I think it depends on the coach. My 96 has been with the same coach for six years. At the end of every season you need to evaluate and decide what is right for you.

There can't be any answer that would fit everyone. Some kids can learn from the same coach forever.
I've never been in a situation where the coaches hand-off teams at specific intervals. I think that's probably a good thing. Ala Texans, the coaches would 'specialize' from ages 11-13 or so, then, 14-15 another coach, 16-18 another...

We were going to have that with Andro, but..that didn't exactly work out. We just ended a six-year stint with one coach - not by our choice, the coach decided he preferred to get paid and went FCD. I know my son would have learned from Josema until he 'graduated' from select. Every year, month, week, day that he didn't learn something was not the coach's fault, it was his own.

I'd say every 3 years sounds about right to me for the majority, it bites for kids and parents who like the status quo, but it's probably better to experience other coaching.