But it doesn't work like that. You might reserve 20% of the time to chores, but you don't have to use all of that 20% every week. Efficiency of maintenance will

But it doesn't work like that. You might reserve 20% of the time to chores, but you don't have to use all of that 20% every week. Efficiency of maintenance will improve after you get your processes and tooling in order and everyone has had a bit of experience with them, and the leftover time can be used for whatever the developers feel most deserves the extra time. The reason to do maintenance constantly is practice, which makes the difference between taking weeks or days vs. hours. When the time spent is evenly distributed, it's also less likely that an important task would be blocked by unavoidable maintenance.