It's not a hard limit, but on the other hand it is an agreed amount of time between stakeholders which carries weight and expectations. Creative work does tend to defy fitting into time constraints. I'm not saying it's impossible to harmonize agreed time and creative work, but an agreed time constraint will be impactful in the creative work itself. We've all seen so many instances of excellent series turning average because they dragged for too long, or terrible endings because they ran out of time or had the carpet pulled out of their feet.