Not 100% sure but i believe it can actually be detected in one's system 18 months. Like i said i might be wrong but i remember reading that somewhere a few years ago.
Now just cause it's detectable doesn't mean there is enough there to continue making gains like one makes while cycling. So after your last shot you can probably still get 2-3 weeks worth of gains. Then everything else after that will totaly depend on you; pct, diet, and training.