The obvious answer is to wait as long as possible. At a minimum I'd wait a week.

Maye a bit late with this reply, but may I suggest you use an automotive primer before applying urethane topcoats.  This will make the top coats uniform, improve adhesion, color hold out and actually require less top coat material.  You will also find that your sanded nitrate base does not look as good as it feels, revealing numerous surface irregularities.  The idea is to block sand the initial primer application to barely reveal the clear nitrate base.  The next coat should be sanded back only to reveal a constant, smooth gray surface with no sand through spots to the nitrate base. I like Duplicolor Fill'n Sand for this process, though others will work as well.  You do not need a urethane primer as lacquer base will do.   Good luck...


Joe