Well, it's been imminent since the summer of 1956 ( http://en.wikipedia.org/wiki/Dartmouth_Conferences )
Never mind the Moore's Law putting computers more capable than our brains less than 30 years away.
http://www.extremetech.com/wp-content/uploads/2013/08/CPU-Sc...
The green line was the only one still going, and it plateaued about a year ago (you'd see that in a newer graph).
But that's no reason to think that it'll take more than 100 years.
We could probably already build a computer more powerful than the human brain, it would just be huge (millions of cores or something). But that wouldn't help, because the real issue is that the Von Neumann architecture fundamentally prevents scaling the kinds of computations we want to do for neural networks. We need something more neuromorphic, although probably still discrete. (I'm guessing basically just a giant DSP integrated into memory on the same core.)
Now, we have increasingly good computational models of human cognition, with abundant limitary results on both human and mechanical reasoning, but only madmen believe AGI will work in their lifetimes.
Irony!