If you really have to multilayer, then as Chris says, normally 24 hours. Just make sure surface is very clean before doing so, or you will just introduce swirls. Either wash car again, if kept outside and freshly contaminated, or prep each panel with some quick detailer spray if in garage.
I've once applied 2 coats of Collinite 476 for extra protection but had to do it in the same 5 hour detail. I'd normally use the Z wax with its natural oils and such, but 476 contains different solvents which can affect the first coat of wax if you layer it on top so soon. Spit shining is the act of spraying the first coat of wax with refridgerated QD spray or equivalent, then layering the second coat on top of this. The cold QD spray then acts as a barrier between the two coats. Buff the second layer of wax without dissolving or hazing the first!
The term apparently originated in the area of Army boot perfection if I remember, where the idea was to get several layers on to the boots for max shine. Hence actual spitting in between to prevent each new layer affecting each underlying layer. This is very apt in the case of 476 because it actually smells like shoe polish rather than a natural oiled proper carnauba wax.
http://www.detailingworld.co.uk/forum/showthread.php?t=8752&highlight=spit
Check the Blackfire product info on PB's site just to check what the score is with the BF though.
If you are in no hurry to multilayer then prob just apply a new layer after a proper wash and dry say every 6-8 weeks or so.
Hope this helps:icon_thumright: