According to Google maps it is about 240 miles (gives or take). From Orlando city centre to Miami city centre.
The driving time in a car would be about 4 hours (or a bit more).
A coach will be longer as it will probably stop off a few times on the way, and may not get able to go at the same speed as a car.
My total guess it would be more like 5 or 6 hours by coach (allowing for stops, slower speed etc).
Note you can do this yourself in Google maps. Just put "Orlando" in Google search, then choose maps. You will see a map of Orlando.
Right click and select "Directions from here"
Now move the map down and find Miami. Then right click and select "Directions to here".
It gives you the route, mileage. and rough driving time.