The number of hearts will add up by one every 24 hours from when you begin your subscription.
For example, if your subscription started on January 1st, 2:00 PM, you will see "♡+1". After 24 hours, at 2:00 PM on January 2nd, the number of hearts will change to "♡+2".

The number of hearts will continue to add up as long as you maintain your subscription without stopping it.
Once your subscription expires, the number will be reset, and it will start from one if you start subscribing again.