A recent study using a productivity app called DeskTime analysed the activity data of its 5.5 million daily logs and narrowed down the 10% of most productive workers to ascertain what the best work/rest ratio is. They found that the most productive employees engaged in work related tasks for 52 minutes then took a 17-minute break.
That 17-minute break is long enough for your brain to disengage long enough to help you feel refreshed and ready to dive back into work for another 52 minutes of productivity. That refreshing disengage is also not so long that you lose focus and derail, but gives you enough of a break that you do not slow yourself down with sloppy mistakes.
This method only works if during that 52 minutes of work you really jump in with both feet and commit to being as productive as possible. The other key is that during your “down time” you mentally disengage from work and do something non-work related like taking a walk around the block or having a quick coffee break with a co-worker.