This is a good question - I dont think I have a good answer. I lean towards not implementing a check or stop your use. Maybe a monthly reminder for one / two months saying you have not paid. After that I would just assume that the user has stopped using my editor.
In order to justify a monthly payment I feel you would really have to be pushing out frequent high quality updates that just kept making it better and better.
If development started to get stagnant I would probably "forget" to renew it after my CC expired or something like that. Or at some point I'd just feel "I've given this guy enough money" and cancel even if I kept using the editor.
