If I were to nitpick, I'd say 60 years isn't completely accurate, but that's certainly around when it ramped up.

I'd say it depends on the western nation. If we were talking the US/Canada, that's probably around right give or take a decade. If you're talking Aus/NZ, I'd easily add 20 years to the number, and if you're talking about the UK, then I wouldn't entirely argue against a century as being the early start of it. I'd say it was definitely early days, since mass immigration wasn't exactly that prevalent in the 1920s, but to deny the early examples of it is also silly.

You'd also have to do a similar breakdown of Europe. There's a big difference between the mass immigration rates of countries like Germany and Poland.