Any time you've grown in your job and you're not receiving a corresponding raise, is a great moment to look around.

But also: I like change. I notice I'm more productive in the first half year on a project. Almost every project is interesting at first. Not all projects are still interesting after a year or two. When I get bored, I know it's time to move on. Some companies may have enough diversity in work, and enough flexibility to move me around, that can keep growing without quitting, but my experience is that most don't.

But I'm not constantly job hunting; as long as I'm enjoying myself, I tend to ignore other offers.