we time our kids in the shower, and if they dont turn the water off when we tell them, then they have to deal with us walking in ( and seeing them nude) and turning it off for them, if we let them shower as long as they want they will be in there for 30 minutes, we give them 5 minutes only, and with all the issues of water running out at the moment its a great way to let them know you just dont waste water.

we used to let the 3 boys shower together, and the 3 girls, but they all play games and end up sqirting all the water out the window, as the opposite sex is outside the window stirring them on, and know one got washed!!