I am checking around here but on Tableau Server - Data Driven alerts can be configured to be checked at any desired frequency per the instructions here - Set Up a Server for Data-Driven Alerts - Tableau
and set to a defaults of every 60 minutes. Are you not finding Tableau Online checking at least once every 60 minutes? If not, please open a support ticket with Tableau Support.
It should work as Patrick described, also remember that when the alert triggers (on the hourly schedule) the alert will not be sent again until it has "gone false" on a later scheduled check.
You may also consider the subscription option 'don't send when view is empty'.
You may create a filter that is equal to your alert condition.
Whenever it is met, the view not blank; when not met, view blank.
Another advantage of this is the subscription schedule will be fixed, e.g. every 15 minutes.
Compared to alert will be relative to the last check time.
Thanks for the inputs.
After some monitoring I realized the alert is indeed checked every 60 minutes.
Thanks for the suggestion.
I don't see the option to schedule subscriptions every 15 minutes. The smallest grain is hourly, which would not make a difference compared to an alert. I guess this is a limitation of Tableau Online - they don't let you refresh your data more frequently.