LOL. When I ask a question like that when I interview someone, its because I can’t think of anything else and I’m stalling for time.
If they say 50, I'm good with that, as that is the mathematical number. If they give a really high number, that indicates to me people who are clueless. I also avoid the, "maybe 40, I'm not very lucky." I don't want those "woe is me" type people.