It depends a lot on your field and what part of the country you're in.
When I lived in the People's Republic of California, it wasn't considered at all unusual for someone to change jobs every two or three years, sometimes more often. Marketing and advertising people, of whom I was one, often job-hopped more than engineers and accounting types.
I did a fair amount of job-hopping, although it was never my goal. I left one company as soon as I saw the handwriting on the wall, (it went under about six months later,) and another because my "supervisor" was psychotic. I sat out the dot.com bust in a job I didn't like much, and in so doing, outlasted a co-worker who made life miserable for everyone in the group. After he finally got fired, things improved considerably. If I hadn't retired, I probably would have stayed there another year or two.