When you are unable to receive your mail then it is returned back to the sender, but the time that it takes for the mail to be returned to the sender varies according to the services that were purchased for the mail such as the mail class used for sending the mail, Change of Address or Hold Mail. So, depending on these the mail will be returned to the sender accordingly.
Sometimes, if the customer is not available to receive the mail then carrier leaves a notice and the item is stored at a local Post Office for a specified amount of time.
So, it is not possible to give an estimated amount of time that when will the mail be returned to the sender, but it will be returned to the sender in the same mail class that was used to send it.
Unfortunately, no notification is provided if a mail is returned to the sender!