1 song. Sometimes you gotta hear it on repeat.
More often it'll be an album and I might play it twice in a row.
For an actual playlist of different songs, I only have two that I listen to frequently;
One with hundreds of personal favourites.
And another with new(or just new to me) stuff I'm interested in hearing, or want to hear again after a long time. I'll listen through and remove stuff if I don't care to hear it again. Sometimes music stays on there for months until I'm "done" with it. And the length fluctuates anywhere from around 20 to 2000 songs or so.

Yes. Unless it's the rare occasion I want to play something competitive like CSGO then I'd use m&k.