The default is to alert when the trigger matches, and again when the service succeeds. So if you flap between 79% and 81% disk utilization, you'll receive alerts as the thresholds are reached.
However, you want reminders after an alert has been triggered. You can use the reminder functionality within Monit to do this.
Here's a sample config from Monit:
For example if you want to be notified each tenth cycle if a service remains in a failed state, you can use:
alert foo@bar with reminder on 10 cycles
Likewise if you want to be notified on each failed cycle, you can use:
alert foo@bar with reminder on 1 cycle
You can extend your stanza with this language:
check filesystem datafs with path /dev/sda1
if space usage > 80% for 5 times within 15 cycles then alert
But you'll have to modify your global
set alert statement or change it at the check stanza level:
set alert firstname.lastname@example.org with reminder on 5 cycles