I did three cycles with little downtime between them - as long as I was on, felt great and kept growing.
then came down with normal clomid taper - but I still feel like I'm missing test, even though my balls are back to normal.
also I have zits all over my upper arms and shoulders.
if I do cycles again, then I'll take the time off in between. I think the resultant crash you get (whatever weight loss and lowered sex drive) will be lessened as well that way.
but then again, as long as you stay on and don't mind increasing your doses, killing your wallet and maybe you liver... it is nice and fun.