It would probably take at current birthrates, and with little or no immigration, about 200 years for it to happen.
Of course, if we returned to pre-1965 Immigration policies, add about 200,000 Ukranians, etc, to the mix and it would never happen. So, it's NOT inevitable, nor it is particularly desirable, contrary to the gushings of Bill Clinton, that Whitey becomes a minority in the US.