The Media Run Time is the total length of a video multiplied by the number of views. (Attention: no multiplication if you filter this metric - the basic value is taken for this case!)
This value is stored in the mt2 parameter of the media request. During implementation it is important to ensure that the length of the video is specified in seconds in the mt2 parameter the very first time the media player is set to “play” (mk parameter) for a specific media ID (mi parameter).
Like categories in page requests, the mt2 parameter of the respective media ID is only evaluated at the very first time the mk parameter is set to “play”. So, if no run time is specified at this time, the Media Run Time in the data will always output 0 - even if the mt2 parameter has been passed with the correct value at a later time.
You can correct an incorrectly transmitted mt2 value afterwards. Since the mt2 is treated internally as a category, you can correct it like any other category: https://support.webtrekk.com/hc/en-us/articles/115004079525-How-can-I-replace-missing-values-in-categories-
For media playback without a fixed playing time, e.g. a livestream, it is common practice to pass the mt2 parameter with the value "0".
Information on the Media Run Time
Documentation and download of Media Tracking