The most upsetting experience in my career thus far was being belittled by an incompetent project manager.
"How could it possibly take that long? How do you not know how long it will take?"
The happiest times of my career have been when we're lacking a PM -- coincidentally the most successful!
Well, if they really said that, you can just dismiss the comment as incompetent. A witty retort may be in order if a non-technical manager is in earshot, e.g.:
"How long would it take for you to earn a green belt in karate?"
"Hmm, what does a green belt entail?"
"Exactly, you don't even know what you don't know, yet."
I don't think this will score you a lot of points.
"We don't know because we've never done this before, and we can only compare to the most similar work we've done, and any existing data."
Before I could say more, they I interrupted: "You have to know, it's your job, or you don't know what your doing," or something along those lines. It was the most heated I've felt. I know what I'd reply with today, but I was younger and more naive then.
Or just offer over-inflated estimates since they won't accept something realistic anyway.
> "or you don't know what your doing,"
That though was unprofessional, and should be addressed as such.
Also, someone asking how could something take so long is not a belittling question. I don't see any problem with either question.