I think it's a gray area, and requires balance. Just because someone hasn't used the app in a month doesn't mean they forgot about it. Too frequent notification

I think it's a gray area, and requires balance. Just because someone hasn't used the app in a month doesn't mean they forgot about it. Too frequent notifications like this would be obnoxious. On the other hand, if they did forget about it, then you should definitely let them know they're still paying for it. Honestly, the best way to go about it might just be to split the subscription model into monthly payments (even if it's an annual subscription, for instance) and then send a notification whenever they're charged. That way it's the same reminder, but it's more of "hey, here's something for your records, we've taken your money" instead of "hey, hey, poke poke, use our app".