Ah, it sort of depends of what I'm doing, and the time of year. I have over 70 playlists on my iPod, so it's pretty inconsistent. I used to make Top 20 playlists every three months, and listen to them regularly, but I kind of got out of the habit of doing that. The thing with short playlists is, while I can get through them during the duration of a school day's being in the hallways/on lunch break, it kind of can risk exhausting the songs in them. The two most recent playlists I've made on my iPod have both 140 songs in them, and that's a pretty good length for me, I think. 70-ish is probably my average, if you don't count a few outlying playlists that I have containing, like, 8 songs, or my entire iPod's contents.
Note I also put songs that I just added into their own playlist, regardless of how few or much of them I add at a time. Re-listening to songs that I got in summertime a few years ago, makes me nostalgic, especially since it's currently summer. d: Haven't been able to add songs to my iPod since January, though. So anything I've downloaded after that, I've just been listening to on my computer in separate WMP playlists, lol.