In these situations I shift the conversation to what is the PM really after. I explain that I can tell them any estimate they want (or they can make it up on their own - no need to even discuss), but does that really do them any good when it is missed? There is usually some pushback at first, but when items are delivered within estimate a few times in a row they realize it's better to take whatever hit up front than it is to miss and adjust.

Also, someone asking how could something take so long is not a belittling question. I don't see any problem with either question.