Choose from these work cycles or make one of your own:

1. The ideal time-box: For every hour of work, take a 5-10 minute break.
2. The healthy time-box: For every 55 minutes of work at the workstation, stretch your body for 5 minutes.
3. Chetan Surpur's 30-30 work cycle: 30 minutes rest after 30 minutes of work. Works for coders.
4. The Pomodoro method of 25 minutes of working at a task with all concentration and then taking 5 minutes break.

The general consensus says not to work for more than 1 hour at a stretch. Use what works for you.