It's impossible to create AGI until we understand the human intelligence completely. At least to know its parameters, to be able to make AI stronger. So far we are nowhere close. Probably, 100 years required to reach that level of understanding.
We suck at estimating in general. In tech we suck bigtime! Whether it takes 50 or 500 years, the point is that we'll totally missestimate it and when it does finally happen it will totally take us by surprise... and likely we'll do something very very stupid in response to being surprised by it.
I'm not sure this is necessarily true. At a smaller scale, we didn't need to understand how humans play chess in order to build a good chess AI. I do think we need a better understanding of what we mean when we talk about intelligence, if we expect to recreate it. Francois Chollet has a good recent paper about this [1].
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