I used to take a week off every 12 weeks. After evaluating my training logs, I am looking at 1 week off for every 6-8 weeks on. I always dread taking the week off, but after I do, all my joints feel better, I have WAY more energy, and am strong as a MOFO. I usually try and do some sort of exercise during the week off though for "active recovery". This could be soccer, snowaboarding, cardio, whatever.