I clear anytime after the the Moss BMC green is dry to the touch , so yes it could be a hour or so if the paint is dry to touch or the next day, i try to clear when it not so humid, but if the clear (I use Duplicolor engine hi heat clear) does turn chauky white looking, not to worry it will clear out as it dries.
Doug, I always clear when using Moss' engine paints as they will bleach out if coolant ever gets on them, and the duplicolor will not, unfortunately the BMC green is not easy to find, so many of use the Moss stuff.