When to change a research focus
I know of no university that advises you to take a sabbatical. Lucky people have departments that allow them to take a sabbatical every once in a while. Sabbaticals don't generally result in a change in research focus.

I think there are three indicators about when it is time to change research focus.


You are bored and want to change
You can no longer publish in your area and receive rejections along the lines of "This is no longer a relevant area"
You can no longer get funding in you area and receive rejections along the lines of "This is no longer a relevant area"