Ask HN: How long is too long?

I've been in my current dev job for about 4 years, moving a C++ fat client to .NET/web (mainly .NET dev and team lead prior to that). I'm probably the one with the most experience/knowledge of the "newer technologies" in the office, but compared to your average hacker in a startup, I'm just a beginner.

Most of what I'm learning is what I'm teaching myself, although I'm learning the odd bit about C++ from colleagues (easily my weakest area).

I live in Berlin, so the startup scene is pretty good, and the prospect of working with a bunch of "like minded" talented people really excites me. On the other hand, I'm gaining experience in web (specifically JS, angular), and I've run training sessions for the company, which I find a really good way to get a deeper understanding of a language or technology. Being the "go-to" guy for questions & architecture certainly has a feel-good factor, even if I still feel like a beginner.

This will be my 5th year, and I'm weighing up staying on, learning-by-doing (as opposed to from colleagues) as much as I can, or trying the startup scene.

5 years feels like a limit - I don't want to stagnate, but I enjoy what I do.

How long do you consider to be too long? i.e. if you read a CV (EDIT: resume), how long would give you a negative feeling about the candidate?