I always heard the theory of time on=time off, though in all honesty I have never known one person who uses AAS to actually follow it...Usually they will be back on sooner then later.
I am Natural now so I don't have this issue, but I do run legal PH's..and I have a protocol where I will do them every 30 days..this gives my body time to clear out and receptors a chance to clear...but often enough to keep a solid gain without using AAS.
I have heard that if you run a good detox following a cycle, your receptors can fully clear in 30-60 days. I never detoxed and never tried this, but an Endo with a very good reputation told me this.