It's about 3 hours if you do not hit a lot of traffic near Tampa. We recently had to drive from WDW to a friend's B-day party in Cape Coral which is a bit further away from i-75 than the airport and did it just under 3 hours. It's about 200 miles door to door.
If you make good time 3 hrs. If it get's congested near Tampa - 3.5 hrs. But even then, you start to feel "closer" to WDW by then and it isn't that bad. Ocassionally we've made it under 3 hrs as well. Depends on how badly the guy behind me is riding my tail