By 2019 half of the children 18 and under will be minorities.
Within five years of an amnesty passing Whites will drop to a minority.
“By 2039 half of the population of the US will be minorities as defined by the USG.”
Thus, it will take much more than 30 years for “minorities” to be a majority of the electorate. 50 years sounds about right.
BTW, all this is assumes that recent immigration patterns stay the same; any change in our immigration laws and enforcement thereof could change things considerably.