11 year old's showers are now 40 minutes. The tank ran dry twice this week

He used to be in and out in five. Since the holidays began it's forty minutes, with music. We're on tankers for May like the rest of the colony, and twice this week the overhead tank was empty by 9 am. I've asked. 'I'm thinking.' I've timed him. He says the timer is wrong. I don't need a lecture on boys this age, I know what age this is. I want a rule that works and that I can enforce without standing outside the bathroom door.