Generally, I think the window is 6 months in advance. Normally, I wait to do my car rentals between 60-90 days ahead bc the deals have been posted (eg: if you go online now, you'll see that there is a summer upgrade deal). However, since my rental for this trip is one-way from Pt. Canaveral to the airport, I wanted to be sure that they would not run out of cars eligible for one-way. I have run into that before, where there would be no problem if it was a r/t rental, but they would not rent one-way.
FYI -- if you are picking up and returning to the airport, and you belong to
AAA, they have gotten me some outstanding deals thru Hertz with their discount.