Ok, I'll go out on a limb but there really is no scientific reason for time on = time off; without monitoring of Test levels, it's just a safety guideline that's been practiced for years. Time off really depends on the reason for the PCT, in the first place. Most guys, particularly those who start young (18-21), need to stay on it longer so as not to upset the hypothalamic-adenhohypophysial axis (I'll skip the hardcore science) Let's just say by utilizing a two steps forward, one step back AAS philosophy, you will allow maximal recovery and have greater opportunity for maximal genetic potential. (12-16 weeks is plenty). Older (late 20's-35) and more experienced BB's are doing it to maximize effectiveness of later cycles (6-8 weeks is fine for most, but some BB's on high tren/test dosages will require double) their reason is wrongfully referred to as "cleaning receptors" but the idea is the same. And us really old guys (40+), whose natural test is in the dumps only need to do it when we need to minimize cholesterol/HDL and other sides, (we're just on replacement therapy, most of us doing < 1 gm/week anyway)