[Geoff Bacon]

I only have a few audio tracks so don't compute running time from the audio. (I get the time by a stop watch on my phone when I'm rehearsing)

I initially used the Mobile Sheets companion to determine which songs didn't have a duration (click on the Duration column header to sort; blank fields are shown at the bottom of the list; click again and they will be at the top of the screen). I only needed durations for a couple of playing groups (each has their own collection).
You can use the companion filter to restrict the collections being considered (this reduces the number of songs to look at).

I suspect there is a way to do this using MSP but I can understand how the companion works.

Now that most songs have a duration, I tend to always fill in the duration field when I add a new song.
When creating a setlist, I regularly check the running time to see if a song is not included e.g. 10/11 means that one of the recently added songs needs a duration.

Hope this helps.
Geoff
PS When setting the duration, you don't have to be to the exact second - add 5-10 seconds to allow for song switching and/or interaction with audience