The advice here is to sit for 20-30 minutes at a time and then take a 2-minute walking break. This sort of schedule happens to correspond nicely with the Pomodoro Technique, which suggests 25 minutes of work followed by 5-minute breaks.
The point is that (1) most people are in fact going to slack off for at least that much time, and (2) doing so in a controlled way is actually beneficial to your performance when not slacking off.
The only evidence I know for these is anecdotal, but I'll take a substantial body of anecdotal evidence over "sounds a bit too much to me" any day.
http://en.wikipedia.org/wiki/Pomodoro_Technique