Well, this depends on how people will use the system. Say, you're just tracking down a used car with best possible price. So you sign up for text notifications so that you're the first one - and once you bought the car, you no longer need these notifications.

Other usage patterns might assume regular, monthly usage.

Since I just launched this and have only a handful of users, I'm yet to see how it'll be used.

Edit: I misread your comment. They will be using this, in the best case scenario, regularly on ongoing basis - so not "monthly" in a sense of regular subscription services.