I love a long, hot shower.  It's one of my favorite things (I fantasize about walk-in showers with multiple shower heads and a seating bench).  But I also live in a studio apartment that doesn't require much in the way of heating and I do all sorts of other things to reduce my impact on the world (no car, compost/recycle).  I kind of figure that some things are going to slide, and in the grand scheme of things, this one doesn't seem so bad.

Also, what about a compromise?  For example:  she can take her long hot showers on weekends, but during the week, she should keep it to 10 minutes or less.

You could also start charging her $5 for every minute over 10 minutes or something like that - especially if you want to drive home the economics of hot water heating.

The Market Based Solution: Time your daughters showers and charge her $1 for every minute past 5 that the shower is running, $2 for every minute after 10, $3 for every minute after 20.  A 10 minute shower would cost $5, 20 minutes would cost $25, 30 minute shower would cost her $55.

Rationing: Turn off the water valve for water flowing out of the hot water heater.  Do this after her 5 minutes is up.  I would suspect that if her hot shower turns cold without warning a few times she may get the picture.  It may work best to not do it every time, but rather be erratic and unpredictable.

I recall the Alexander Portnoy spent a lot of time in the bathroom too.  There may be other reasons teens sequester themselves naked in the bathroom other than hygienic ones.

Try turning on the hot water faucets somewhere else in the house - this will make the shower a chilling experience, especially if the faucet is on a lower level than the shower.

If you shower in a bathtub here's is what I do. I quickly wash my hair. Then I close the bathtub drain while I condition and wash my body. In my case, I'm usually finished before I have an inch of water around my feet.

So I can shower as long as I want, knowing that I'm not wasting energy -- the heat from the hot water will warm up the bathroom the rest of the day (not noticeably, but that's what physics tells us).

Of course, this method won't work for summer.

...or more appropriately, showered in it.  At the decidedly non-futuristic Internationaal Zeemanshuis (Seaman's House) in Antwerp, Belgium the showers operated like the sinks in many American public restrooms.  That is, you pushed a button down, and got a gradually diminishing stream of water that lasted for approx. 30 seconds.  Need more water?  Push the button again.

Exactly.  I'd wager that many of us used the bathroom for various onanistic pursuits...those tub faucets can be a girl's best friend.

You seem like the perfect candidates for solar thermal water heating system. Solar thermal uses the power of the sun to heat your water. It is much cheaper than PVC solar panels as it does not require the same complicated battery technology, so it's very cost-effective. I think they only cost ~$200 depending on the size of your water tank and will quickly help pay for itself and save energy.

But if you're worried about wasted water in addition to energy, which, depending on your state, may be a big issue, you should also take your daughter to Good Vibes--or maybe just tell her about it... :)

Comprimises and good, direct, honest, two-way conversations with her are highly recommended, as are giving her choices. You don't want to be too heavy-handed on these issues I think (I was a teenage girl not too long ago), but rather encourage her to think about her choices and even possible mitigation strategies.

But you should definately investigate solar thermal water heating. Everyone who owns their house or plans on staying a few years should.

I have a 24yo stepson living with me and his mother.  He not only takes long showers, but usually twice a day as well. He complained when I replaced the full flow shower head with a 2.5gpm  unit but has learned to 'live with it'. It's not worth the hassle to argue beyond that. IMHO

Sorry if someone already posted this idea, I'm in a hurry:

I have a low-flow NAVY showerhead, which has a button on it that you can use to reduce or stop the flow.  I use it to reduce flow to keep some steam going while I lather/shampoo/shave (which actually makes it EASIER, since water isn't washing away your lather or shaving cream or whatnot).  It's a lot easier than trying to avoid the flow/splash, and you don't need the water on full blast to keep yourself warm.  It's really convenient to be able to control the flow without inadvertently changing temp, which most showers make it hard to do!  

Umbra, because that comment was right on. I raised 4 kids, now safely launched, and first having a heart to heart talk is best - mutual respect. And remembering that they are just passing through and to enjoy them as they are is the best advice there is. It's amazing what they will remember later on when they're not doing the proving their independence thing. I have had the pleasure of hearing my kids tell their kids almost word for word some of my admonitions. Put it out there with respect and it will come back. . . and keep the peace.

Some kids will listen to a heart-to-heart and others will just roll their eyes. My stepdaughter (now 18) is in the latter category, and uses every opportunity to negate our environmentally conscious behaviors: she buys her own industrial laundry soap so she won't have to use our "green" brand; she tosses trash on the sidewalk or in the street; keeps the thermostat in her room set at 80 degrees, day and night, all winter; takes 35-minute showers and frequent hot baths; goes through three rolls of toilet paper a day (for what I don't want to know); and never turns off lights or the television. Talking to her doesn't change anything, her behavior is part of her extended teenage rebellion that she's inflicted on us since age 12. In cases like that, one approach is to make her pay for her energy use. She gets a monthly child support check from her stepfather, and we recently started deducting a portion of it to cover her extravagant energy use (our electricity bill doubled when she moved in with us). That got her attention.

...buy one with a smaller capacity! That way, the hot water will run out after 5 or 10 minutes and she'll be forced to get out of the shower!

I wonder whether some long showerers genuinely believe that they need that long to get clean. If you watch how she washes dishes, does she labour over getting them completely spotless? Is she a perfectionist in other areas?

I worked for a while in a nursing home once, showering elderly people among other things, and quickly found that different staff members had different ideas about what should be done.  This greatly affected how long the showers took!

Perhaps you could talk about this with your daughter. If she feels hurried out of the shower in a way that leaves her feeling unclean for the rest of the day, that might not be the best solution. Relaxing her high standard of cleanliness (if this is the case) might be helpful.

Here's what I do to keep my son from taking too-long showers:

Before he starts, I fill a 48 oz. plastic cup with cold water, and set it on the sink.  I tell him he has x minutes to take his shower,then I'm coming in and pouring the water on his head (no door lock to worry about).  Guess what?  It works.

You could also do what my brother and I did when we were younger and wanted the other out of the shower...just go flush a toilet in the house.  That gets 'em out of the shower pretty quick.

Hey! This was my question, and I enjoyed Umbra's answers, but kind of feel like I opened up Pandora's box on the feedback.

We live in Seattle and - of course! -- keep the house temp dialed down low, so who wouldn't want to linger in a warm shower - it's the warmest place in the house. Also, we unfortunately inherited a monster-size hot water heater installed by the previous owners; one of these days that's gonna have to go, but in the meantime, the hot water never, ever runs out.

Anyway, I've already talked it over with my daughter, who's a good kid and completely on-board with environmental concerns (already takes the bus everywhere, for example - just hates being cold). And we all like the lowest-tech timer out there - the five-minute egg timer that attaches to the bathroom tiles with a suction cup looks like a winner to me...it might not bring her to a total stop, but it's simple and easy and a good reminder to everyone else in the family, too.

I'm an 18 year old girl. Someone already mentioned a similar proposition, but for a different reason.

What I do is I close the drain of my bathtub while I shower (If you have only a shower I remember when I was little we had this rubbery circle that we placed over the drain and it stopped the drain). Once I'm done soaping, shampooing, and conditioning I turn off the shower and use the water that collected to shave. I also soap up while I let my conditioner soak in a bit. I don't know how much time my shower takes but it is sure less than 10 minutes... probably about 5.

I am 15 and love relaxing and taking hot showers in the morning to wake up. What I do is turn the water off when soaping up and shampooing/conditioning, just like when I'm brushing my teeth. This way I don't waste the hot water, save energy, and am pretty quick (15mins). And as the third child with two older sisters and a little brother, I have to be quick and save the hot water.