To cure before polishing? Pretty quick- it can be polished out after a day or two. But I woudn't use Future as a top clearcoat for a car body. If it is applied thick enough to polish out, it tends to haze, craze, and/or crack, sometimes weeks after being applied.
Tamiya X22 is
made for this application, and is far superior. Well worth the incredible cost per ounce difference. It too can be polished out in a day or two, but it won't haze, craze, or crack.
See here for an example.
http://www.automotiveforums.com/vbul...d.php?t=401357
The best use for Future is to dip clear parts (windshields, headlights, etc) in. Save yourself the agrivation, and don't try it as a clearcoat.