Dennett and Hofstadter have "extremely large" estimates of the time to intelligent machines as well. I expect such estimates will prove to be wrong - but it is true that we don't know much about the size of the target in the search space - or how rough that space is - so almost any estimate is defensible.
Eliezer Yudkowsky and Scott Aaronson - Percontations: Artificial Intelligence and Quantum Mechanics
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