The hairspray itself can be left for days (at least) but in my experience the paint applied over it (the chipping colour) must be attacked fairly soon. I use Tamiya acrylics and have found that if I allow the paint to cure, it becomes much more difficult to "break through" it to the hairspray. Adding a retarder to the acrylic certainly helps, but I still find it necessary to start chipping within 15 or 20 minutes.
Also, I think that the sooner you start, the easier it is to vary the size of the chips, as you can use the curing of the acrylic to you advantage.
I have tried chipping cured Tamiya with Windex (ammonia), but with little success.
So, in consideration of all this, after the whole model is hair sprayed, I apply the paint coat to only one area at a time - do the chipping, then more on to another area.
Remember too that you can achieve different size chips by varying the thickness of the hair spray coat - a thin gives finer chips. That's why I decant my hair spray and apply it with an airbrush, for better control. And you must have complete coverage with the hair spray layer, otherwise the colour layer will never come off.
Good luck - practice on a paint mule first.
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