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If a feather is completely gone, shaft and all, it will start to grow back immediately. If the feather is only broken, but the shaft is still intact (attached) then it won't grow back until the bird molts.
If pecking is a problem, you can purchase a product called pine tar - you'll find it at the feed stores, in the horse section. It's messy to apply and sticks to everything, but chickens hate the taste of it and it usually cures pecking in just one use.
To help your girl(s) grow back their feathers quicker, you can up their intake of protein. Some good ways to increase protein - switch to a gamebird feed (be sure and offer oyster shell on the side if you do this, for the calcium), give them black oil sunflower seeds as a regular treat, offer other treats that are high in protein, and occasional treats of dry cat food.
Basicaly feathers are protein, so any treat you can think of that has a high protein content will help.
Finally, consider purchasing a hen saddle for your bare hen. It will protect her skin while the feathers grow back.