I've found 1-2 years is also about the time it takes for me to become bored and want a new project, new responsibilities (promotion), etc (I've only asked for a raise once during my entire career, so it usually isn't about the money when I leave, though I do usually get a nice pay bump for switching companies).

I'll ask for what I want, invariably get told some version of "no", and start looking for a new job.