How do you know this is the right job for you if you've just started at it? How do you know that you won't be bored at this job or you won't be cleaning others' mess, after a few days/months (No pun intended).
Of course, there could also be a quagmire of corporate politics and sloth that won't get revealed for months. If that happens, I'll reevaluate periodically and open the door to change.
Even in the best case, I don't expect I'll work there for the rest of my life. 5 or 10 years would be fine and all I look for right now.
[Edit: added more thoughts below]
I think the crux is learning about yourself and what you want out of a job, and out of life. That takes time, it's a nonlinear process, and it's non-deterministic too. Make mistakes (embrace them, not fear them), be prepared for change (keep a few month's worth of cash in the bank), follow your dreams, and do the things you love as much as possible.
And try everything. Live life to it's fullest.
"Everything in excess! To enjoy the flavor of life, take big bites. Moderation is for monks." - Lazarus Long (RAH).