I constantly churn on this question. I have been in the same profession (or at least manufacturing trades/tech/engineering at different times), but I've never managed to stay with one employer long enough to break the 3 week vacation time barrier - sometimes by choice, life circumstances or company failing - it's just worked out that way.
In 2003, I decided I had had enough. Wasn't married, no kids, a little savings and I wanted to travel. First time I quit a job in my life, without a new job to walk into. It was great. I figured I had enough skills - from trades to engineering - to land on my feet. I was 43. I spent next year traveling - not just on motorcycle. After that I spent a year working for myself, through a friend, doing some CAD work (architectural). When that dried up, I landed the job I currently have - back to the cube in engineering. It's been 5 years now and I'm ready for my next move.
My take is this; if you have the time and the means, take the time. Especially in your 20's. You have the rest of your life to have a career. You may find that having a "career" is nice, but it's not like that, in and of itself, determines success or happiness. Especially in todays age of turnover, I wouldn't worry too much about not having longevity at any one workplace on my resume.
2007 BMW 650 XChallenge..
2009 BMW R1200 GSA..(Gone..
2004 BMW R1150 GSA (Gone..
"You think dogs will not be in heaven? I tell you, they will be there long before any of us." Robert Louis Stevenson