I assume you have quite a few users with different expiration dates. So yes, you'll generate a unique subscription key for every user.
You can generate one subscription key for all users -- but then it will expire for all users at the same time.
Does this help? Just let me know if you need more information.
BTW, here are two interesting links:
http://www.lindersoft.com/forums/showthread.php?t=1191
http://www.lindersoft.com/forums/showthread.php?t=2895
Friedrich