I'd shoot for at least 1.5 years in each position, with a few positions holding on for 3 or longer.
Times are changing. My dad has worked for 5 companies over his career, but the last 3 were effectively the same (Company A, then Company B was spunoff by A, then Company C acquired A). I've been in the workforce for about 3.5 years now, and am already on my second company. I'll be looking to jump in another half a year or so, mostly because that is how you can get the bigger salary bumps (the first jump was a $20k increase, although COL increased as well).
As a young employee, I don't think it looks bad. If you're in your 40's, it probably is starting to look worse and worse. I think a great part of being a mustachian is that you likely won't get to the point that it will start to look worse and worse.