I went from St. Pete to Key West, door-to-door in 6 hours flat...
You could do it in about 4 at the speed limit, 3.5 pushing it a little.
Keep in mind, once you leave Miami, US1 is bounces between 35-45mph with an occasional 55.
Mapquest sucks for ETA's. It said It should take me like 9.5 hours...ya, okay.