If you wanna push it you would go for lets say 4 weeks on 4 weeks off. But I would recommend doing more than the time you were on. lets say if you were on for 3 weeks id go off for about 5 to maybe even 6. Thats just me though, im not a bodybuilder, just looking for an edge. Its like anything your body produces.. ie creatine. You cycle it because (studies have not yet shown, but to be safe) you dont want your body to stop natural production. It will stop producing creatine because your body will now rely on the supplement rather than natural production (in theory). But PH is different, I would recommend no more than 3 cycles a year, this is very toxic to your liver and can be very detrimental if you do not know what you're doing. Keep in mind, im not a doctor, this is according to information I have gathered.