Every month or two a survey like this is posted here. This is the first one I recall where every question was clearly worded and one that I could answer.
I'm not sure how the survey could help answer if and when electronic books will replace paper ones, but that's not my problem. Personally, I think that newly printed paper books will eventually be as rare as the book type they replaced (hand-written ones on animal skin). When that will happen is unknowable. How long did the transition to the printing press take?