When my son first started his vision therapy, he was supposed to do a particular task ten minutes a day. I could divide those ten minutes into two five-minute chunks, or three sets of about three minutes. (I could probably even have done five sets of two minutes, but the likelihood of me remembering to do something five times over the course of the day was vanishingly small.)
The first time my son did his vision therapy, he worked for five minutes, and seemed to do just fine. But then he had a headache the rest of the day.