Once you've finished learning anything new, and your job is becoming significantly more routine, start looking around for the next job (with a pay rise). Rather

Once you've finished learning anything new, and your job is becoming significantly more routine, start looking around for the next job (with a pay rise). Rather than learn new things on your own a few hours a week, you can get paid (more) to learn new things at work for many hours a week. Three significantly different jobs over ten years will make you far more competent than the same job for fifteen or even twenty years. If you get to five years in the same job, take a long hard look at yourself and check that you are actually still learning and still making yourself more valuable and still improving (which I would expect to really only happen if you weren't actually doing the same job as you were in the first of those five years).