Me: "Five years"

PM: "Five years?!?! Why is it going to take you that long?"

Me: "It probably won't, but as I already told you, we're going into a territory with an untold number of unknowns, and I don't want to give you any false hopes about an unrealistically short deadline. If you would like me to lie to you, please make that request in an email so I can have a timestamp."

PM: *Leaves, red in the face, so I can get back to work before the next useless interruption.*