How long into an episode a typical listener listens is a metric most podcasters are interested in knowing about their podcasts. However, you won't find Length of Play or Length of Listen as a defined in the IAB Podcast Measurement Technical Guidelines (the industry standard on podcast measurement). Here's why:
Podcasts are listened to in a distributed environment. That means that the listener chooses the phone app (e.g Apple Podcasts) or online player they will use to listen to the podcast. It is at the app/player level that data such as plays and listening time is accessible. Some players track this, others do not. They may or may not make that data available to the podcaster. Most do not.
Podtrac measurement works via counting accesses to the RSS feed that delivers the podcast - regardless of which app/player is being used by the listener. You can learn more about how that works in our Measurement white paper.
But the bottom line is that play data and listening length data is not a metric available in podcasting. The industry standard is to use downloads and audience which are available on the Podtrac dashboard.