I can usually only stand things being the same for around two years. Then, if I don't shake it up somehow, I start to feel more and more blaaaaaagh.

That said, I've lived with the same partner for seventeen years and I also don't seem to be bored with my toddler yet! You don't need to change everything to move things along a bit. Try reviewing your actual goals, not just career-based ones. Actually look at what sort of life you would like to be leading in five years, ten years, twenty years... I've even found it useful to examine my hundred year goals, the world I'd like my daughter to exist in. It's really clarified some of my environmental and political responsibilities. Once you've done that, see if the actions you're taking are getting you closer to these goals. If you feel you're stagnating, I'm guessing that at least some of your goals are being neglected.

My other advice would be to read Your Money or Your Life. Even people who don't plan to leave the workforce can benefit from an examination of why they're working.