If it's working well, then theoretically you don't need any logs!
It's when things start NOT working well, you need to have enough log files to diagnose & fix the issue.
Provided your server's file system has regular back-ups - including the log files, which can therefore be retrieved if required - you should only need the most recent log files (couple of days, a week at the most). But this does assume you have a filesystem backup of older log files.
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