Well call me nobody.
Look at the Fiberglassing and Smoothing tutorial sticky.
The short answer is that you paint it when you feel it has been sculpted and sanded to your liking.
You've been making a lot of help topics lately that have all been asked and answered before and are now all recorded in the
Pepakura Ultimate FAQs sticky.
Please consult it before starting more new threads.
I'm sorry if I seem a little rough about it. I've just seen almost 3 forum pages of common help threads spring up in the last few days. (not all you by any means, but I think you'll find your answers more quickly this way)