Until the generation that is driving it dies off, so probably ~20 years.

I'm being flippant, but--look at the vote splits by age. Young voters in the UK overwhelmingly voted against Brexit. Young voters in the U.S. overwhelmingly voted against Trump.

The voice of populism today is the voice of economic and cultural disenfranchisement. It is difficult to re-employ millions of 60-year-old folks whose skills are out of date. It is difficult to get 60-year-old folks who have spent their lives in a homogeneous town, to accept multicultural diversity.

They are crying out for help and success again, and we'll continue down this course as a society until we either give it to them, or they leave us.