Those with authority to change user groups will have the option of specifying a expiry time for those permissions. Users who can add a particular right, but not remove it, will be able to lengthen the duration of a user's existing group membership (for example, if they are reconfirmed by their community for another year), but not shorten it (because reducing expiry to e.g. 1 second is tantamount to removing the right). On the other hand, those who can remove a right but not add it will not gain any additional powers. The feature only applies to local groups; no changes are being made to global groups for the moment.
The primary audience for this change is Meta stewards, but other communities will be able to make use of it if they wish. The ability to grant user permissions is restricted to users with advanced rights, and these users are expected to follow policy, so if a community wishes to restrict local administrators/bureaucrats from granting temporary user rights, policy controls are an appropriate solution.
If you have bureaucrat or steward rights on the beta cluster, you can head over there to test the user rights expiry interface right now.
This will fulfil the #10 wish on the 2016 Community Wishlist Survey.