After the 5-10 hours a week when you are "in the zone" you start borrowing time. It's possible to sustain productivity for significantly longer than that, but

After the 5-10 hours a week when you are "in the zone" you start borrowing time. It's possible to sustain productivity for significantly longer than that, but it comes at the expense of rest, which leads to burnout. When you burn out, you no longer get 5-10 hours of focus per week; that time goes to recovery from last week.