Few diff ways to skin the car. How Wrivest mentioned, working below %70 of 1RM of your primary lift, is one way. You can also keep the intensity up (%80-90 of 1RM) but greatly reduce the volume.
As for my assistance work, I'll either turn the volume or intensity down just a tad. I just won't go balls to the wall. It really doesn't change all that much. You can still be building muscle or burning cals.
As for when to deload, you should deload before you say "Damn, I really need to deload" You're not going to get any weaker. In fact, you'll most likely come back stronged.
Depending on your programming, you could deload every 4-6 weeks.
The only fully weeks OFF from the gym I'll take, is vacation time.