Monitoring rules are built this way that they have full flexibility and support for (almost) everything.
Rules can be configured to check on newly created records, updated records or on any record found within a specific interval.
This interval can be configured per rule and supports minutes, hours and days.
Finally with the "Comparison Type" you can say if the rule should go into error based on "more than", "more than or equal to", "equal to", "less than", "less than or equal to" than the "Number of records".
Don't worry if it seems complex though! With any change you make on the rule we explain the rule in human-readable text so you get exactly what it does at the "Rule explained" label.
Here's a sample of the configuration of a monitor rule:

As you can configure required actions on a monitoring rule it means you can do pretty much everything.
This hooks into the default Odoo server actions which means you can link multiple actions to one rule.
For example: do you want to send an e-mail to person X and an SMS to person Y while assigning an activity Z?
Just link three actions to the monitoring rule and they all get their own notification(s)!