NET10 Service Days
I have over 5000 minutes on my NET10 phone. I have to buy more minutes to get service days. I don't need minutes, just service days.
Tracfone offers a service that extends your service period for 30 days for $6 a month, but NET10 does not. So, unfortunately, even though you don't need the airtime, the only way to extend your service on NET10 is to buy more pay as you go airtime cards. The cheapest of these, as you probably know, is to get the $30 cards that extend your service for 60 days and add 300 minutes.
If you want to spend less per month, you should consider switching to a different plan such as Tracfone or T-Mobile, however, you would lose those 5000 minutes.