If I feel that I need it, I will usually take 1 week off every 6-8 weeks, if I get to that point and feel fine, I will keep going.
The most time I will really take off is 2 weeks, if I take more than that, then I start to freak out and get antsy and start to really miss working out. I find that I really don't lose much size or strength, guess its just a mental thing for me.