If you want to modify a particular workflow to send such notifications, you can use the workflow "Send Notification" (under Library > Mail) and invoke it at the end of your workflow's success/failure branches.
Or perhaps you want to monitor all/any workflow executions within whole vRO server? In this case, you could try to write a custom long-running workflow that will periodically retrieve all workflow tokens, enumerate those of them which completed within a given time interval, and again use "Send Notification" workflow to send a report which executions were completed or failed.
