Depends on your needs and time of the year for your stay. As many have said, sooner is better.

We made our reservations almost a year in advance.

Florida State Parks allow reservations up to 11 months in advance. For the popular parks like this one, you should book that far in advance if possible. But there's always a chance you can find a cancellation later if you didn't reserve that far in advance.

Busy place. Earliest possible via their online portal. I really don't know the details of how often they have vacancy but the park is beautiful but not that large so I imagine high demand.