When I was in portfolio school, I was lucky enough to have Ernie Schenck as my mentor. (If you're new to the blog, you can read some of his sage advice here, here and here.)
When I took my first job out of school, I wrote him about it. I'm posting his response, because it's going to apply to you, too:
Greg-- That's great. Good for you. It sounds like a real good opportunity to get your career off the ground. Just remember to stay vigilant. Protect your brand. Not that you have one yet, but pretend that you do:) If things are working out and you're getting your needs met, and you know what those are at this point, then great. If not, though, don't wait to jump ship. Trust me on this inertia thing. The longer people stay in one place the harder it is for them to leave. Just keep your wits about you. Be in touch. --Ernie Schenck