4-6 weeks is generally long enough for any cycle, depending on how long you run it and what you are using. But I have never heard of anyone running a 12 week pct or even close to that.
As others said, 4-6 weeks is fine. I've ran 4 week PCT with every cycle i've done and each time i've lost barely any weight, and levels returned to normal. Of course, I had proper PCT, if you don't have a good one setup, your setting yourself up for some real troubles.