Without traffic, about 5 hours. However, you've got a better chance of hitting the lottery!
Between road construction & people having accidents, it took us 7 hours one way and 6 hours the other earlier this month. Actually, we started from about 20 minutes or so away from wdw, so add 20 minutes to my actual times.
I think the distance is 312 miles, but as Sue says with traffic a reasonable guess is 7 hours. Try to avoid Jacksonville at rush hour, it can be a real pig !!!