I'm in a similar position. I still like working for my company, but I don't feel like I'm contributing to the depth I desire. My recommendations fall on deaf ears, the politics never change, and I feel my career stagnating. That's how I know it's probably time to go.
It's especially painful because I've been here going on five years, I was the first employee and the company has met success largely upon my shoulders. I'm still here because we're not planning on staying in Columbus much longer, and I can use the freedom I'm allowed to pursue educational opportunities and personal growth.
But even that is nearing the end of it's novelty, I'm ready for new challenges. I think when you're ready for new challenges, new faces, and a refreshed outlook on your career - that's when it's time to go.