Ask: How do you decide when to take a break?

The question mostly directed at developers that are working for themselves either as a consultant or an entrepreneur. There is no shortage of work to be done when you are working for yourself, so when do you decide its time to take a break?

What are you working patterns within a day and a week? How long do you work before you decide that its time to take a short break or even a holiday? And does this decision strategy change when working on different tasks?

I am just curious how other developers decide.