A week off is a vitally important aspect to your training. This will allow your body to recover from the physical stress it has endured over the past training cycle. Without this crucial recovery period, overtraining can be an unfortunate side effect.
As a general rule of thumb, I recommend taking a week's break after 6-10 consecutive weeks of training. 6-10 weeks is highly variable, but so too are our bodies. Sometimes after 8 weeks you will still feel great and be experiencing some great results. However sometimes after 6 weeks you can just feel absolutely worn out.
Another factor in determining when a week off is due, is based upon your results. If your weight gains slow consistently, your fitness levels plateau or your fat loss comes to a halt, this may be an indication that it is time for a break.
The beauty about the week off is that it allows you to recover physically and mentally. However it also assists you in refocussing on your goals for the next period of training. You can then go back into the gym feeling pumped, focussed and ready to train with some major intensity.