If everything is under your own control (no layoffs), I would say the sweet spot is between 2 and 5 years. There was a post on HN talked about how after 2 years, your salary will be held back compare to switching. I added a maximum of 5 years before you MUST change job in order to stay relevant in the industry.

Changing too often can be perceived as unloyal, but changing not often enough can be perceived as unable to adapt to new technology.