In reply to OP, I was banned as well and had a large amount of days of premium on my account the day of the ban. I doubt that any of it will be given back when my account is unblocked, and I doubt that it will be that way for anyone else. Technically speaking, it would be difficult to determine exactly how many days of premium each and every banned account had before the time of their ban, so I wouldn't expect your premium to be handed back to you.
I would recommend, though, buying a large pack of JMBs when you get unblocked. Those are known to give out a lot of premium days (you may already know this but I might as well still mention it), so you still have an option to get your premium days 'back' in a sense.