If you have an inside contact at the company I would say apply when you are within 20% of the posted minimums. There is always flexibility when you have an internal recommendation.

Even without a rec, Spirit is looking for a lot of qualified people right now. If you are willing to live in (or are from) San Juan then your chances go through the roof.

Competitive applicants would have 121 experience and approx 3000 hours. Spirit is highly interested in people who have knowledge of "cerebral" aircraft like the A320 series. The machine thinks. Spirit wants people who will manage that machine and relate to it on a level of intelligence rather than wit.

That being said, get familiar with complex FMS systems and glass cockpit aircraft if you want to "stand apart" from the others. From your position as an ATR FO it looks like you wont be flying glass anytime soon; so think of creative ways to acquire that experience to show management that 1) you CAN do it and 2) you did it for Spirit.

Hope that helps.