I don't really have an answer to the "how long" - although I agree that an allover large-ish stipple is a pretty quick way to do it. I am going to suggest that you invest in (or dig out from wherever it is) a portable tape/CD player with headphones, and listen to some books on tape (from the library) while you work. Music is great too, of course, but sometimes getting hooked into a good plot can really make the time fly!
Alison