@Johannes_Schoen , the size and the percentage of space are predictable values, the duration is not, as it dependent on the volume of logging which may vary drastically based on the complexity of your policy, number of users, depth of logging and numerous other factors.
Your management server may not even be capable of retaining said 30 days of logging, in which case this setting will be moot.
This said, there is some discussion going on about merits of enabling it and improving the logging configuration:
https://community.checkpoint.com/t5/Logging-and-Reporting/How-to-store-logs-Focus-on-time-range-inst...
There are also two workarounds mentioned, one script-based and another using GUIDBEDIT.
Yet another option is rotating files at Midnight and forwarding them to external server.
Forwarding will be purging local logs.
You are still stuck with the retention limit problem on the target server though.