I think it's usually the same as the cancellation policy would be for the dates at that property on a paid stay and if you book it on the phone they will tell you the policy or if you book it online the confirmation you get should tell you.

I've had to cancel a couple (with plenty of notice) and never had any problems. I just had to call HHonors after I cancelled to ask them to void the certificdates and deposit the points into my account. Unlike most of the airlines, hilton doesn't charge you to cancel or modify an awards reservation and to redposit points back into your account. I guess I bet not give them any ideas