About an hour, hour and a half..depending on traffic. Morning rush hour traffic is terrible from the airport getting onto I4, and evening traffic is terrible from the airport getting on to I4. If you arrive anywhere from 10a - 3p, you should be fine. Evening traffic starts to pick up around 3:30/4p, so my advice is to get your rental car asap (the airport isn't that large) and head out to WDW as soon as you can. Once you're stuck in traffic trying to get onto I4, you will be at a dead stop for a little while.
E_mail me if you need help getting around or want locations for grocery stores or whatever. (this goes for all of you flying into Tampa..this is my home town!)