Overnight is good here in wyoming where our humidity is in the negative digits, but two or three days would be better if you have a relative humidity of 25% or more. The easiest way to tell is after the paint is dry to the touch, it will still feel cool to the touch until all of the moisture has evaporated out of it. When you can't feel a difference between the piece you are painting and the temp of the room around it, go ahead and clear cote. I also highly recommend hitting your color coat with a light sanding with 0000 grade steel wool to smooth out the irregularities inherant to spray paint. After that, you can wet sand with the same grade steel wool to make the surface as smooth as glass. Then, rinse the part off VERY well, and towel it dry with a chamois or a lint free towel. Always remember to tack cloth the piece immediately before clear-coating to remove any lint or dust blebs that may have collected on it. I also like to use three to four coats of clear-coat to enhance the color.
Good luck!! post some pics up here when your done, and if need anything else, don't hesititate to PM me.