Mine were going to expire in June with no hope of flying F9 by then so I did the 2,000 mile purchase for $50 and got 3,000 total with their bonus offer. The miles posted immediately along with the new expiration date 6 months from now = October. Any kind of activity on the account extends the 6 months.
Spirit miles expire in 3 months if you think Frontier is bad. I can usually fly Frontier every 6 months but Spirit and 3 months is ridiculous so I've given up on them.