I am planning to take a Spirit flight where I have not prepaid for a seat assignment. I will let Spirit pick my seat for me, which happens when I check in.
From personal experience and from a few questionable sources, it seems like Spirit assigns seats at the back of the plane first (and possibly the middle seats first as well):
I haven't been able to find out much else via Google...
Both times where I have checked in as soon as possible (24 hours before), I ended up with a middle seat near the back of the plane. In the times where I waited longer (1-3 hours before), I have been able to get aisle and window seats. Of course, this is a very small sample size, so I don't know if it has just been a coincidence or if this is indicative of how their "random" seat assignment algorithm works.
Question: Is there an optimal time to check into a Spirit flight to maximize my chances of getting an aisle or window seat? If this is not possible, does anybody know if later check-ins will tend to seat me closer to the front of the plane?
EDIT: In case anybody is curious, I have two data points. I checked in ~3 hours before a flight and ended up with 7B, which is near the front but a middle seat. I also checked in ~16 hours before a flight and got 17F.