I don't really have long playlists, I think it defeats the purpose of it in a way because you listen to a certain playlist to hear songs (or a certain category of songs) and anything over 70+ songs is way too much for me to get through. I usually listen to playlists that last around 30-90 minutes; each one varying.
So I don't really have long playlists, but I have around 20 different playlists at the moment. Sometimes though I usually listen to individuals albums rather than creating unique playlists. If we're talking total amount of songs though, they're all over different devices. I'd say anything over 2000 to guesstimate, but be bothered actually counting.