I have a customer table in my database. This table contains customer information including his/her balance.

Now I want to add his/her balance after every 30 days depends on what promo or plan he/she applied

example: he applied for a plan 1599 so after every 30 days his balance must add the price of the plan he applied.

current balance = 0
after 30 days balance = 1599

How will i do this?