Free Spirit, the frequent flyer program for Florida-based budget carrier Spirit Airlines, is very controversial. On the one hand, it is very restrictive. Your miles expire after only three months of inactivity, unless you have the Bank of America Spirit Airlines World MasterCard (see below).  Most programs give you at least a year, and some up to two years. On the other hand, Free Spirit miles can give you amazing and unbeatable value if you’re in the right place at the right time.