it can last from several days to a month or two. My girl is around 10 months now, and I have noticed she is quilling again. They may be more irritable, scratching from discomfort of new quills growing in, but as long as they are not losing quills in large patches (and are not getting bald spots) then I wouldn't worry about it. Dry skin is also common in hedgehogs, so that wouldn't be something I would be majorly concerned about. You can give it a warm bath using Aveno or some kind of oatmeal based solution that will help sooth the skin, also will help dry skin, etc.
If you suspect mites, you can take a black cloth and rub it on your hedgie, see if you can see any small white dots moving around, or another sign is tattered ears, crust around the quills, and quill loss in patches. If you have a cooperative vet you can just tell them that you need some kitten revolution, and tell them your kitten or puppy has fleas or something - it would be cheaper than bringing your hedgehog in for a skin scraping and exam, if you already know it has mites.
I would think it is just quilling though, but try and get a close look at the skin and see if you can see anything - usually they're not that visible unless its extreme though.
