Honestly as long as a company has enough variety internally I (slightly) prefer staying with until they "go bad".

Now I also prefer startups and my carrier isn't yet that old (like ~8 years) so never is still the answer.

Through companies being often way less likely to give a trusted long term employee a payrise or trust them with more responsibilities then they are to do so with a new hired employee they have hard times to properly judge the skill of is still an (absurd but not IT specific) issue.