By default, when a time off request is submitted by an employee, their manager will see a workflow notice within their To Do's section advising that there is a new timesheet which is awaiting approval:

The manager will then also see the time off booking in their calendar:

The manager will also receive an email notification telling them that one of their employees has submitted a time off request.

 By default, if the manager does not action the request i.e. Approve or Decline, they will receive two further reminders. The first reminder will be 3 days after the submitted date and the second reminder will be 7 days after the time off request submitted date.

The settings for these reminders can not be changed or modified, but can be set in a workflow as escalations which is why you can disable the default reminder option. To do this go to:

Administration -> Company -> Settings -> Company settings -> Notifications

 Then select 'Disabled' from the 'Default time off approval reminders' dropdown.