I always assume people are really asking "how long and why?". It's likely they're looking for some context around the number of days, because reading the opinions of people who have similar interests is helpful when planning travel. But I think people should actually list they're interests when they ask. If someone says, I like touring old churches and cathedrals and eating at nice restaurants, it's easy to answer with "In x, you should see these three churches, and here are a couple great restaurants to try...you should be able to do all this in 2 days".

I've typically found when I list my specific interests, I get some very useful responses in the Destinations forum.