Ask HN: Engineers, how do you decide when it's time to quit?

I've been at my company for a few years, and I've become a domain expert in many areas of the business that people rely on. I enjoy my coworkers and have great hours as an engineer, but I'm paid 20% under market rate and feel like I'm not really being challenged (unless I go out of my way to create a challenge for myself).

I feel as though I'm growing content with my position, and don't want to leave unless I find something perfect. Am I being naive in my desires?

How do you decide when it's time to leave a business that you, for all intents-and-purposes enjoy growing with? (this is a throwaway account as my coworkers know my normal account)