I've admittedly been on both sides of this discussion. If someone asks, I'll usually tell them the amount of time I'm gone with qualifiers (such as I was hauling butt downhill). Then it's up to them to adjust to fit their situation.

When I'm asking, it's usually either a) a polite way to make small talk when you pass someone on the trail or b) in rare cases to determine whether or not I should give up on my goal and look for a place to make camp earlier. Usually I have a pretty good idea of where I am based on maps though.