How often should I take a week off from training?

There really isn't a standard rule. You need to keep a detailed training diary and find out when you start to stop seeing improvements.

For me, depending on the workout, it will be anywhere from 6 weeks (Max-OT) to 16 weeks (HST or other exercise with built-in active rest weeks). Once you start dreading the exercise or visiting the gym or stop seeing strength gains, etc, it's probably a good idea to take a short break.