subscription service

Here is a case:
site have some nodes for which users are ready to pay something. but the only pay option should be with time limit. i mean - when user decides to pay for particular content (node) he subscribes for lets say - 3 months. After that period of time access to that node should be again diened.
Adding specific role to specific user manually is not a problem. The trouble is with removing access (role) from exipred users.
Of course there is option to do it manually just by removing roles from expired users. But if users are far too many to accomplish it?

So any ideas how to do it? And it's ok that assigning roles goes manually. Just if there could be an option for role like an expirition time.

I hope You got what I meant :)

Thnx.