For paid applications getting a monthly bill is a great reminder. If we are talking about free application, where subscription == registration, take caution and

For paid applications getting a monthly bill is a great reminder. If we are talking about free application, where subscription == registration, take caution and inform only users that actually have confirmed their email and actually did some meaningful interactions with your app. I tend to forget all apps I've subscribed to when frantically searching for a tool or something similar, or use it once and forget, because my needs have changed. The correct way would be to inform user about account being unused for a year or so and then delete all its data, if user does not take any action.