For sure, Spirit’s reward program is not the one you will earn much with. They have less an unremarkable sign-up bonus, let me say even, not up to par when most airlines offer 50,000. It has an annual fee and doesn’t offer cool airline specific bonuses that you would normally expect from airlines. And the feedback on Spirit is generally negative, so you want to consider other rewards programs before sticking with this one.
What we would suggest is actually getting a universal travel card. They give you rewards on any kinds of purchase, and you can redeem them for Spirit flights. Travel cards actually let you earn faster than any airline cards would. Take, for example, the Discover it® Miles. This is a fine alternative to the Spirit Card. You earn 1.5 miles on all purchases and at the end of the first year you get the same number that you’ve earned so far (provided you are a new cardmember).
There’s no annual fee, the miles are applicable to all travel purchases, no just Spirit, and they never expire.