I have been using a Pomodoro app that allows you to adjust the work segment s well as the break segment. So far I have done this: Work for 25 minutes followed by a 5 minute break, another 25 minutes with a 5 minute break. And if I go another 25minutes I take a 10-15 minute break. That has worked very well for me--bunkered down with no distractions, with phone on airplane mode.