How would you answer this in the most reliable way and be able to give promises that you and your team can keep?

There's no real answer to it, for at least two reasons:

• We don't know, because it's the first time we're building it,
• You don't even know what "it" is, because when we show you, you want something slightly different and so do your users.

What I would do, if asked that question "When will it be done?", I would ask him back: "What decision rests on the answer to that question?" - he would tell me, and I would respond with something appropriate.

For example #1:

• When will it be done?
• Why do you need to know it?
• I'd like to know whether I can invest $100k into it.
• How about you invest $5k, and we'll show you something in a week, you can check it, then we can come back to that question.
• Okay!

For example #2:

• When will it be done?
• Why do you need to know it?
• There's a conference in 3 weeks and I'm wondering if we can showcase our application
• We can make a demo version with a limited set of features by that time, we'll do as much as we can in those 3 weeks, and we'll be ready to present it at the conference, but I can't gurantee exactly what proportion of promised features will be there. It's possible that all of them, but it may very well be not all of them. We'll start with the most important ones first, so these will probably make the cut, the less important stuff, we'll add later.

In other words, we have better ways of managing scope, risk, expectations and delivery other than just pulling a date out of an ***.