I really depends on I-4. If you catch I-4 at a really good time about 20m minutes. If during ruch hour 1-3 hours. Ruch hours are normally from 7 - 9 am and 3:30 - 7 pm. I was told that there is a lunch rush hour but I have never seen it. The time also depends if there is an accident, for some reason if there is one people only want to drive 2 MPH.
I avoid I-4 and take turkey lake road which turns into palm parkway, it brings you to lake buena vista. I say about 10 - 15 minutes to get to downtown disney area.