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perhaps:

1) a set of timers- days, weeks, months, as appropriate for your subscription units.

2) a data model with users and subscriptions and a means of cross-referencing them. subscriptions are time-stamped upon creation...

3) a set of routines that perhaps runs daily that compares now (Date().getTime()) with subscription timestamps then compare to timer-values and mark subscriptions as desired "expired" or maybe "one week before expiring" and thus worthy of an email etc...

4) a means of notifying the client of a users subscription status. (additional field on published user object etc?)

5) a means of restricting assets, probably in the router, based upon #4...

If you are asking these questions, you need a technical co-founder. You really, really need one.