[buro9]

Whenever the job no longer challenges / interests me.

Which in my early career, when personal growth was mostly on the technical front, was every couple of years... but in my later career, when personal growth was mostly soft skills, complexity and nuance, has been more in the ~6 year region.

I think the important question is whether you're still learning, and whether you find joy in the environment and people, when the answer starts to be no you should move on (though I understand a lot of people stay put if it aligns to their financial priorities but I just quit and go make myself happy instead)