@Etoile-sama: I did powder between the layers at first, though that was out of necessity because I was doing my own paint and I couldn't move around without fear of sticking to myself if I wasn't powdered. I do recommend letting each layer dry before repainting, although it's generally safe to repaint when the layer is almost dry. If you don't let it dry at least a little, the paint will streak. Again the hair dryer works pretty good for drying fast. But it doesn't seem to affect the quality of the paint whether or not you powder between layers.
I used the no-tack PAX, but I don't think it really matters, as I had to powder anyway >< The no-tack PAX doesn't stick to non-painted surfaces when dry, but it sticks to itself, so it rather defeats the purpose.
It took me 6 hours, although most of that was because it was my first time doing full body paint. Next time I do it, it will probably be faster XD
As for the eye area, I actually just applied it on to my eyes and eyelids. I suppose that if you wear PAX a lot, it's not safe, but this was a one-time deal so it seemed fine. If you don't want to paint it around your eyes (it IS uncomfortable and your eyelids stick together if you paint them) you might want to look into Mehron or other face paint brands that make water-based paints that are safe for eyes. Since it's such a small area it shouldn't matter too much if it rubs off a little since you can reapply.
@Lucrezea: Actually, it didn't feel bad at all! I actually started forgetting that I was painted once we reached the con. Part of it was that the rest of my costume was rather uncomfortable (really heavy wig + 6 in shoes + 3 inch fingernails) so I was more focused on my head and feet aching. But also part of it was also the paint did really feel fine. It felt very breathable and I didn't have any uh...negative reaction to it.
I do, as Verdatum says above, highly recommend you try the paint out on different parts of the body before the big day, so that if you do suffer an allergic reaction to it, there will be a smaller amount to remove and you will have time to find something else that your skin doesn't dislike.