Hi Scott, your a master instructor for a reason and I don't doubt you. But if a person can't focus more than 20 minutes on something they want to master, maybe that deep desire isn't there like they thought. I'd have to set a timer for 20 minutes to get me to stop. When I often look at the time I'm usually on the table a couple more hours than I thought.
Most students (mental or physical activities) need study breaks. It's not "Scientists said don't play pool for more than 20 minutes" but "Scientists agree, take a five-minute break every 20 to 50 minutes or so to refresh, and to allow learning to seep in."
Most players have had a breakthrough up to 24 hours after a session. For example, the next day after an emotional match when you realize you hit the ball too hard or etc. to do your best.
If you benefit by what is known as "falling in", getting into a zone where you can play and learn for hours without a break, that's great--and one way to know what you're good at doing.
When I'm booked for an all-day lesson I institute regular rest breaks and also check in with the player to see if we're doing too many rest breaks, or too few.