Whats the purpose? Do I really need 90 days or prune after 30 ?

Do you mean the logs from the Downloads app?

If so, those are for your own reference if needed. You can prune them however you prefer. 

No, logs for the filesystem:

Those are also for your reference (or ours, in the case of a problem where you file a support ticket).

You can set pruning to whatever choice you prefer. 30 days should be fine unless you have a need for a longer period of time.