here is some cron setting examples.

will execute queues hourly:
0 * * * * /path/to/openpne/symfony opMailq:execute

will execute queues every 30 minutes:
0 * * * * /path/to/openpne/symfony opMailq:execute
30 * * * * /path/to/openpne/symfony opMailq:execute

will execute queues every minutes(!!not recommended!!):
* * * * * /path/to/openpne/symfony opMailq:execute