The maximum number of sustainable 90-minute focused learning blocks per day is likely four, with one to three being more realistic for most people.

“The answer to that is probably four. And I say probably because some people have schedules, lifestyles, in which four 90-minute blocks of focused learning is possible per day, but that's highly unusual. For most people, it's going to be one or two, maybe three-- four, I would place in the really extraordinary end of things, maybe if you're cramming for exams or you've managed to go on a writing retreat or a learning and retreat of some sort where you can devote essentially, all of your non-sleeping, non-eating time to learning. But most people simply can't organize their life that way.”