That is a tricky question to answer. Once off my sust cycle, I lost some strength gains and some muscle, but alot has to do with my diet or lack thereof. The most size I had put on was after a primo cycle. I put on a massive amount of weight (to me) and alot of it was water. I still have quite a bit of muscle development but I keep those gains by remaining in the gym and working my tail off!!!!!!!
I you do a cycle, get where you want thand then stay home on the couch and eat ho-ho's and haugendaus, then, yes, you will lose all of your hard work.