Create a new workflow from the template Overdue Approval Requests Workflow and enable it.
Go to Notification setup and add the Overdue entry.
To send the notification, you will need to run Send Overdue Approval Notifications report. This report will check if there is any overdue approval entry (based on the due date) and create a new entry in Overdue Approval Entries table. BC will pick up that entry and will send the notification automatically.
You have the option to run the report manually or you can use job queue entry to run the report on schedule.
Report number is 1509 : “Send Overdue Appr. Notif.”.