Due to stress my parakeet pulled most of its feathers. How long does it take for them to grow back?

I just got a new parakeet and it pulled its tail, chest, and back feathers out. He still has feather on his head and wings. He is still very healthy and playful. He stopped pulling once I bought him some parakeets to share the cage with. How long will Kojack (that what I named him because he is now bald) to grow his feathers back?
Thanks for the replies. I am sorry to say, but the bird broke its leg in the cage and later died.