From the Spirit Airlines 'Free Spirit' website FAQ.
"Can my FREE SPIRIT miles expire?
"Yes they can, but don't worry - they don't have to.
"As long as you earn any miles within the previous three months, your miles won't expire. If you don't meet that requirement though, any miles older than three months will expire.
"The easiest way to earn miles and keep them from expiring is to get our FREE SPIRIT™ MasterCard® and make at least one purchase per month.
"You can also earn more miles by flying on us or using one of our many partners."
Awfully short life-cycle.