At least five inches. Prenatal vitamins are very powerful, and work well. When I was pregnant with my son, my hair grew like crazy, about 5 inches, and that was without the vitamins, so you do the math. Lol. Then again, everyones hair is different, and sometimes those prenatal vitamins are bogus, so don't get your hopes up. My good friend used the vitamins, and nothing happened, everyone is different. One thing to do is get the black ones that say living selfish in bold letters.