By default, you receive notifications about attempts to exceed limits (it there are such attempts) every hour. When you receive a notification, you can look into the reasons why domains are attempting to send more email messages than allowed in their limits.
To change the frequency of notifications about attempts to exceed limits on outgoing mail:
Go to Tools & Settings > Mail Server Settings.
Alternatively, go to Tools & Settings > Outgoing Mail Control > Change Settings (in the Server-wide settings group).
Select the period from the list titled Send an email notification every (period of time). Plesk will analyze statistics for this period and send a notification if necessary.