Add enough each month (actually, each 30-days) to renew the prepaid plan. Depends on which plan you chose and how much it costs.
You can always add more. Available or leftover funds deposited to account will always be used up first when plan renews. There's no limit to what you can prepay, lol, and there's convenience in paying a few months ahead, but beware that once money is deposited into your Self-Serve account it will not be refunded or returned.
You can set up AutoPay (and collect AutoPay Reward plus bonus data) with a credit card if you like. Otherwise you basically have to login to Self-Serve to check (and maybe top up) your balance each time a "Public Mobile here..." payment reminder is sent out.
If you don't pay in time then your plan and service are automatically suspended. They automatically reactivate once payment is received. At Public Mobile there's no fees or costs for any of this, you just won't get phone service until you pay up, although if the plan remains suspended for too long (90 days) then the account will terminate.
