Lauri,
Are you asking how long before you can sand and polish clearcoat? Or color? With catalyzed polyurethane clear coat (top coat), you can usually handle it after 24 hours (though it's usually still pretty soft). You can sand it after 48 hours or so and polish it, though you may want to sand it after 24 hours but wait a day or two more before polishing. The stuff is pretty stable after 48 hours, but takes up to a week to completely cure.