Hello,
it depends on your needs.
Personally I keep all logs for three months with logs spitted by day (to avoid loosing many journal log files due to corruption in case of space trouble) and sometimes do a vaccum if required.
I never had trouble with vaccum.
You can control the retention of your journal log with internal mechanism of journald journald.conf
https://www.freedesktop.org/software/systemd/man/journald.conf.html
Here is an example of my config:
Storage=Persistent
Compress=true
SplitMode=uid
MaxFileSec=1day
MaxRetentionSec=3month
Create your needed *.conf under /etc/systemd/journald.conf.d/ instead of modifying the original journald.conf
Kr,