plenty early
Depending on where/how you're coming in you have options. Rentals from major airports (Orlando or Tampa) will be available at any time. They are usually a few dollars more than a non-airport rental location, but make up for it in convenience if you're doing the airline thing. They bring a small fleet of rental cars into Bartow (KBOW) and Winter Haven (KGIF) if you're flying yourself in. Not sure which vendors, but you can contact the FBO and they can help you out. Other local airports on the Tampa side of SnF probably have the same arrangement. Not sure what is available at SnF, hopefully others will chime in with more info.