From Florida State Parks (Reserve America) reservation rules:
"If you are unable to arrive on the first day of the reservation, the park will hold the reservation until check-out time the next day (1:00 p.m. local time for campsites and 11:00 a.m. for cabins). After that time, if you have not called the park, directly, the park may no-show the reservation, cancel it, and refund the balance, less the first night's use fees."
In my opinion, this is a fair rule. If you don't show, you should forfeit the site and allow someone else to have it. Better yet, if you know you aren't going to make it, call and cancel so someone else can enjoy it.