Technically Tello is every 30 days (annoying that it's not exactly a month but that's prepaid for you). But you can renew or adjust your plan part way through the month and it will roll over your remaining data and minutes and start the timer over again. So if your plan is about to renew and you have a gig of data left over, you can renew manually before the auto renew goes through and keep your leftover data. If you care that much to save a couple bucks.