You're not gonna like this answer I'm pretty sure but although some medical books have stated that Hot Flashes should last from 6 to 24 months at most hot flashes can last for up to 7 years.
Yes that's right you might experience hot flashes for up to 7 years before they go away so be prepared for that.
Although some women I know have only had their hot flashes for 2 years but a few women I know had hot flashes through menopause for 7 years.