Leave whenever it is convenient for you. When you get to the first exit AFTER Jacksonville International Airport, you're at Jacksonville's beltway. Don't take I-295 which is the western beltway. Instead go east on SR 9A which is the eastern beltway and doesn't go as far out of the way. It bypasses downtown and rejoins I-95 south of the city. You'll be in Orlando late morning or early afternoon and should have no problem remaining on I-4 through O-town.
BobK/Orlando