After running a system with journald, the /var/log/journal folder will start to grow in size. It is possible to trim it down by either deleting entries based on date logs. The following example will delete all but the last 31 days of logs:
sudo journalctl --vacuum-time=31d
Or based on size. The following example will delete all execpt for the latest 3 Gibibytes worth of logs:
sudo journalctl --vacuum-size=3G
The above commands can be run on a weekly basis via cron.