When to change a research focus Some say that a person should change the work field every 5 to 7 years in order to stay productive and avoid stagnation. How does this apply to academia, for example, changing a research focus? 

For example, some universities advise to take a sabbatical every 5 years or so.

Edited as suggested: What are the indicators to show that it is time or an opportunity to change the research area? Perhaps, there is a good answer based on your experience?
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"