I'll just throw a round figure out there. "About five years." Maybe "about ten years." Or first thing off the top of my head "about yay long or so."
How about never.
Australia and New Zealand are going to maintain a decent standard of living and intact economies much longer than anywhere else because of their capacity for self-sufficiency. As for the rest of the Western world, it's done. It won't be bouncing back.
Britain, Europe and America have no critical mass of human capital left and to quote Lothrup Stoddard ... we could lose all our bridges, buildings and infrastructure to rubble and yet we would rebuild it in ten years if we still had our best genetic stock. If we lost those sharp faculties of perception and problem solving to dissolution then we could not rebuild any of it given a thousand years.
It took countless millennia of natural selection to produce the high water mark of postwar 1950s America and only a couple generations to lose it all. Once lost, it's not something that is going to come back. Not for a long, long time. There will be a new "normal" and the Kwans will discover it is "normal" for another family of men, not their own.
Just in time, a new Ice Age arrives to demonstrate how nature separates the wheat from the chaff. This is not a man's doing, nor something you can protest or sue against for discrimination. Nature has her own truth and it is different from that of Kwanzanians. Rudyard Kipling's Gods of the Copybooks have opened the page again and are waiting for mankind to take up their pencils for another lesson.