I read some documents and found this:
The current scheduler is poll cycle based. If a service check is scheduled with the every cron statement, Monit will check if the current time match the cron-string pattern. If it does, then the check is performed otherwise it is skipped. The cron specification does not guarantee when exactly the test will run, this depends on the default poll time and the length of the check cycle. In other words, we cannot guarantee that Monit will run on a specific time. Therefor we strongly recommend to use an asterix in the minute field or at minimum a range, e..g. 0-15. Never use a specific minute as Monit may not run on that minute.
How could I find out what's the default poll time and the length of the check cycle?