When I meet with founders, this is one of the first questions I ask. What does success look like in 6, 12, 18 months? In other words, what are your goals, and how will you know you've accomplished them? What do you need to prove in order to make the effort worthwhile? And how big is your vision?
What I'm trying to find out with this question is how you think about the project's future. Some people think too narrowly, some have unrealistic expectations, and some simply haven't considered the question much. If you haven't thought about where you want to go, how can you possibly get there?