These days I don't think there is really a stigma about the length of time you stay at an employer, at least from my experience anyway. I work in a professional field - I guess you'd call it either public policy development or lobbying/advocacy or some combination of the two, although I'm an economist not a lobbyist - and the entire policy team within my unit has turned over in the past 12 months. At the same time, the people who came to replace those who left had been in their previous jobs for a fairly short space of time (from 9 months to 3 years), and the general manager didn't bat an eyelid.
I suppose my point is that it will depend on a) the field you are in and b) your reason for leaving, as was said previously. If you left because you didn't get along with your previous employer because you are/were a dick, or you were fired, then thats probably not great. But if its for career advancement, a new challenge or any other more positive reason that you can justify, I can't see a problem with something like 1 year to 18 months, let alone 3 years. At the same time, I wouldn't make a habit of changing and moving often, as that can ring alarm bells. But once or twice for good reason, can't see a problem.
Personally, I've been in my current job for 12 months now and was recently taken off "Graduate" status, which meant an increase in responsibility, accountability and pay - but I'm basically doing the same stuff I was doing previously. In my field, every day is different and challenging so I can't see myself getting bored or tired or sick of it for the foreseeable future, despite my next "career move" simply being a move from economist to senior economist...which will again entail more accountability, responsibility and pay, but no real change in the type of work that I'm doing.
EDIT: I'm sure theres supposed to be a full stop in there somewhere...