The cron expression for every 6 hours
0 */6 * * *
The hour step */6 selects 0, 6, 12, 18 — four firings per day at 00:00, 06:00, 12:00, 18:00. Six is a divisor of 24, so the cadence is exactly 6 hours apart.
See the dedicated guide for the full breakdown: Cron every hour.
Paste it into the editor above to see the next runs on a calendar.