How long would it take to get to Florida from Manchester airport?

It highly depends where you start from, and if you mean the total trip, or only the hours in the plane.

For reference:

From Miami International to Brussels Airport (only 30-60 minutes flight from Manchester) is 14 hours of flight, and about 22 hours total trip.

This is because there are no direct flights available, and all go via New York, Boston, or Chicago, or few other places.

If you have a direct flight, you can count on about a 11 hours flight, ~10 if you leave from Orlando, and ~9 from Atlanta (North Florida)