Right, that's the point I was going to make too.
As for when it starts, for me it was one of the very first signs that I was pregnant. It started very early and hung around for about the whole first trimester.
It's very individual though. If you know you're pregnant and you haven't experienced nausea yet, that's not necessarily a problem. If you're not pregnant and you've experienced nausea, it doesn't necessarily have anything to do with pregnancy.
Why did you want to know? (Just in terms of providing the info you're actually going for...)