There isn't a set in stone number of weeks that you need to stop at, but at some point, yes, you want to take some time off. Androgen receotors don't down regulate, but in my real world experience, I can tell you that over time, gear becomes less effective. Look at it this way. Your body is always trying to reach homeostasis. Over time, X mg's of test isn't going to do much more than keep your system feeling "normal". You either need to change compounds, add more or take time off. For more expereinced guys, cycles are oftened extended by starting at a dose lower than their target dose and pushing it up later in the cycle. More experienced guys will also switch compounds more often to keep the gains coming. No matter what you do though, no matter how long you play the game of trying to stay "on", there always comes a time that you need a break. I find that after 16 to 20 wks, depending on the gear and doses, it's time for a break. I get labs done every 3 months and my decission to come off or go on, is highly based off of how those labs look. 

For a new or newer user, 12 wks should be plenty of time to make some good, keepable gains. After that, do PCT, take your time off and plan for your next run.