How often do I need to clean the cache?

There is a flag “–cleanup-cache” but I don’t know when should be executed. Every week? every month? only when the application suggest?

For example, I run the commands “backup”, “forget”, “prune” and “check” with “–cache-dir dir” command. If the system never warnings about old cache, do I need to clean it manually?

This command doesn’t touch your currently used cache directories but removes old, unused cache directories. It should be save to run this command at any time. That said I’ve only used it once I think. There should be no need to run this command regularly unless you play around with a lot of repositories and different cache directories.