It's great that they think hygiene is important. It's a hassle that the bathroom is tied up. How do you get a teenager to be faster in the shower?
It depends on why they are taking so much time in the bathroom. It could be that they are just enjoying the shower. Or they could be practicing different looks in front of the mirror. Or they could be enjoying the privacy of the bathroom - most teenagers don't have anywhere else that provides complete privacy with a locked door. Make sure they have those things elsewhere and they may not take as long.
As a temporary solution you could always turn off the hot water tank when they've been in there too long :)
You should sit down and discuss with your son or daughter what the underlying problem is and brainstorm possible solutions.
Is the problem that his siblings are not getting a turn to shower in the morning? Or it's making them late for school? Is it that the siblings need to get in the bathroom to brush their teeth or use the toilet? Is it that there's only one bathroom in the whole house and everyone's having to hold bodily functions when they need to go because he's showing so long? That other family members are getting cold showers because (s)he's used up the hot water? Is he being not curteous or considerate of other family members? Address the underlying issue as the problem when looking for solutions.
There might be a variety of different solutions that would solve the underlying problem. Perhaps you set a schedule where he has to get up earlier if he wants extended time in the shower (or waits till everyone else is done) and a large wall clock in the bathroom so (s)he can keep an eye on the time so they realize when their allotted time slot is up for shower so other family members can use the bathroom. If he isn't good with the schedule, perhaps you need to get an opaque shower curtain (for privacy) and replace the bathroom doorknob with one that doesn't lock (or use a paperclip to unlock the door from the outside), and allow other family members to come up and use the sinks to brush their teeth etc after whatever time his alloted private shower time is over, so he is not disrupting their need to get ready by a certain time. Perhaps you just need to move toothbrushes or blow dryers or contact lenses or makeup or whatever else moved to a bedroom dressing table, another sink, etc. Perhaps your house is ill-equipped for the number of people, and you need to get a plumber to install a sink outside of the bathroom for toothbrushing or put up a partition so the shower is seperated from the sink? Perhaps you need a full-length mirror in a bedroom? Do you need a better hot water heater so more people can have hot showers? Or a rule about how long showers can be unless you're willing to wait to be the last shower, so everyone can have a fair chance to get a hot shower? Is it lack of planning on your part that things you need to get ready are in the bathroom that could be stored elsewhere? Once you realize why him (or her) taking long showers is a problem in the first place, you'll probably get a lot farther in finding an adequate solution.
You can make a deal with your teenager. Short showers in the morning and if he plans on taking longer showers or a bath he has to wait until he's the last one using the bathroom or take the longer shower at night. When he doesn't keep to that rule, you can agree to turn off the hot water as Grant proposed.
If this is about water costs, you can skip the first part and just agree to turn off the hot water of they exceed a certain span of time. Put a cooking clock in the bathroom, so he can hear it's time to turn off the tap or a cold wave is coming his way.
I can tell you if you want to stop your son or kids to take the longest shower ever... I have a hot water shut off system that I design and install... And how it works is that when your kids are in the shower you Control the hot water by a switch ... But it doesn't turn the water off it leaves the cold water on so they get a cold hit of cold shower ... And you'll get that high pitch screams coming from the bathroom every time... lol
The only thing with this system the kids will hate you for it :(
Replace the doorknob with one that doesn't lock. Let the teen know that after a certain number of minutes (during the high-traffic time of day), others will be free to come in and use the facilities -- even to the point of co-showering!
But please, definitely, allow relaxed showering at some other time of day. The relaxation and opportunity for day dreaming, problem solving, etc., are priceless.
I know it sounds simple, but have you tried just explaining to them why you want them to take shorter showers?
It's not guaranteed to work, but for some teenagers, giving a reason for a rule is better than giving the actual rule - knowing that hot water is limited, that everyone else's time needs to be accounted for, and that their own limited time is also an issue, are all reasons that you should be teaching them, not just "take shorter showers because I said so".
Like @Grant said, there may be legitimate reasons why they're taking so long, so you should find that out as well, and if possible find better outlets for those things with them.
Or, if they prefer taking longer showers, and the issue is simply time in the morning, you could encourage them to shower at night. Just be sure to warn them about the dangers of bedhead, and that they dry their hair out thoroughly afterwards.
Turn down the water heater so it only lasts for 15 minutes.