Forever... Seriously, it varies from bird to bird. I have had some that drop just a few feathers and take weeks to regrow them; some drop many feathers and re-grow them quickly. I'd like to think a molt is a one month process, but I think that's being generous. Draft free & good nutrition during the molt...
It really does depend, but if you're concerned be sure to rule out a couple of things: Make sure you don't have a pest problem. Most pests that affect a molt are fleas and mites, critters you will be able to see with your naked eye. Pick through your chicken's feathers and see if you spot any. (Diatomaceous earth will kill them and not harm your chicken.) Is the environment producing stress? Running out of food/water may send a bird into a molt (even very briefly), or they may be lacking nutrition. Anyone pecking on the bird/pulling feathers out? Molts normally begin on the head and work their way down. If it looks out of pattern, this may indicate another issue (see above). Hope that helps a little!
