And yes, the GM can just arbitrarily change whatever depending on how they’re feeling about it. Like is there 1 guard or 10 in the next room? Same thing. They can drain your resources in a thousand different ways, this time issue is no different. If you don’t trust your GM with that then it’s a bad play dynamic anyway.
Another way to solve the time issue could be to count the time when you’re in combat but not out of combat. You could justify it by the stress of combat putting strain on your abilities or something. I know I’d prefer that because having to count 15min intervals constantly would be worse for me than worrying about using up my abilities.