Yes, it can be used off-cycle, or even instead of a cycle if you aren't ready or willing to go that route.
As far as shutdown, there aren't any great studies on that. The one study I've seen with human subjects used only 3 mg per day; a standard dose for a lifter is 10-30 mg per day. Based on what evidence I've seen, I'd say that a cycle of Ostarine will suppress your natural test production, but will not shut you down completely.
I'd say that most people run it 6-10 weeks. Based on my own Ostarine experiment, if I were to run it again I'd use it for 6-8 weeks at 25-30 mg per day.
As far as using it in post cycle therapy (pct), my opinion is that it's a double-edged sword. On the one hand, it will help you hold onto your gains. On the other hand, it could impair your hormonal recovery. Then again, maybe the post cycle therapy (pct) drugs could negate the suppressive effects of Ostarine...
If you want to know more about Ostarine, check out the links I posted in this thread:
Whats the best sarm