St. Surin-Lord noted that while it may take three to six months for the hair to start growing back, it will likely take longer than that for your original, full head of hair to return. Depending on the length of your hair, those new hairs will be playing catch-up until they grow to the length of the rest of the hair on your head. So after losing hair, it may take another six months to several years (if you have long hair) for your hair to fully fill out, St. Surin-Lord said.