iTunes Custom RSS Tags - Duration
Duration is the iTunes tag associated with the 'Time' component of each episode of your podcast. This tag is implemented at the item level, displaying a time value for each episode in the format HH:MM:SS. Since people have few details about your show when they encounter it for the first time, this tag provides information that may help a listener decide whether they have time to tune in for one of your episodes. While it won't hurt anything to leave this tag out, as Apple indicates duration is for informational purposes only, not having it listed displays a Not Available notice in the Time description in iTunes Music Store. After a subscriber downloads any episode from your podcast, the Time is based on the actual audio file header information.
Display the Duration of your individual episode file like this: