"I use T-Mobile pay-as-you-go. I put $50 on it in September and didn't have to refill it until early December. I got about 85 days out of $50. The downside is I don't use data. But the upside is I don't pay for it, either."

Once you top up additional $50 (min for gold prepaid is $100), your expiry date will be 1 year for every refill (even if its only 10 dollars).