You can answer with order of magnitude approximations (days, months, years, might be impossible).
The order of magnitude approximation is good enough, and for ones in which you can't make an order of magnitude approximation you should be able to back it up.
e.g. c.a. 2002 a PMM said "All we need for this idea to work is speaker-independent voice recognition in a noisy environment; how long will that take?"
The correct answer to that question isn't a time estimate, or "I don't know" it's saying that doing so is an R&D problem, not an engineering problem.
The order of magnitude approximation is good enough, and for ones in which you can't make an order of magnitude approximation you should be able to back it up.
e.g. c.a. 2002 a PMM said "All we need for this idea to work is speaker-independent voice recognition in a noisy environment; how long will that take?"
The correct answer to that question isn't a time estimate, or "I don't know" it's saying that doing so is an R&D problem, not an engineering problem.