In general, gains slow down after 6 weeks or so and suppression increases, beginning in "shut down." 6 weeks was standard before the middle 90's. It's also way I believe 4 week cycles are best for solid gains that can be maintained naturally and/or if you want to maintain a properly functioning HPTA. Then again, with the PCT options today, (HCGenerate, UNLEASHED, BRIDGE, etc) I believe you can do 6-8 week cycles and recover completely.