Depends on the individual I think.
My chosen profession is highly technical (software engineer), so the more you get into a comfort zone, the more you aren't being challenged and the more likely you'll find it harder to find a new position.
I've switched companies thus far every time I reach the point where I really know the business and processes very well (ie. just starting to get comfortable). That happens to be about 1.5 - 2 years.
Every time you switch you learn a hell of a lot more and your technical bar increases so much that you walk into an interviewing knowing you'll get an offer.
It is hard to switch tho, you spend a lot of time building up relationships with people and proving yourself, only to do it over again.
Still it really builds your confidence in how easily you could get a job. I mean when I started out, I would send my CV everywhere and hope to get a job. Now I pick a company and apply specifically only at that company (because I want to work there).
Not sure if other professions can go this route also, but I'm very sure that any profession you can learn a lot by switching jobs every 2-3 or so years.