The event log grows with every slowed-down or blocked request, and with every request while Log allowed requests is on.
Automatic cleanup
Events older than Keep events (days) (30 by default) are deleted a thousand at a time, now and then, while the site serves requests. Request counters older than their window are deleted the same way. Set Keep events to 0 to keep every event.
Deleting old events now
On the dashboard, click Delete old events under Old events. Traffic Guard deletes the events older than the retention period (for up to 20 seconds; click again if many remain) and every stale request counter, and says how many events it deleted.
Bans, rules and the allowlist are never deleted by a cleanup.