No, there is no expiration date for your wiki. At the end of the fiscal year, you may be contacted and asked if your wiki is still in use. If you say it is still in use, it will remain available. If you say it is no longer in use, the pages will be archived, and the space deleted.
Yes, we encourage reuse of wiki spaces from one semester to the next. You can add/remove/change user permissions at any point in time.