If you purchase a phone sim cardand you're staying in the USA longer than 30 days you simply top-up at the end of every 30 days. Once you return home, it can be re-used within 90 days, after which time it permanently and forever expires.
If you purchase an iPad, Tablet, USB Modem or Hotspot sim card and you're staying in the USA longer than 30 days you simply recharge at the end of every 30 days. Once you return home, it can be re-used within 365 days, after which time it permanently and forever expires.