I did some research on prepaid plans a while ago and it seems like pretty much all of them require you to buy minutes every so often to keep them from expiring. Usually, a $10-$15 card will renew the plan for 1 month, a $20-$50 card will renew for 3 months, and an $80-$100 card will renew for a whole year.
The minutes on the account will all expire at once, and the expiration date is based on the last time you bought minutes.