I have 5 kids, I hear you loud and clear!
My oldest 2 are 14 and 10, sometime sI stick a 4 year old in with one and a 3 year old in with the other and they shower their buddies, haha!
Anyway, we bought a timer that was safe in the shower and would set it for 5 minutes, showing them how long 5 minutes was and then they had about 30 seconds after it went off to get out for the first few showers. After that they had to be done in the 5 minutes.
It works pretty well and if they aren't my threat was to come in and wash them. They scared them, haha!
Of course they are all boys so it would make then thinks twice before they would want mom to come it.
Another thing you could do it come in after the time you have alotted with a pitcher of ice water and toss it over the top :)
Tell them if they aren't out in ___ minutes they will wish they were. hehe!
I Don't think you are unreasonable about wanting them to take short showers. Just remember to allow for a long tub bath once in awhile for fun.