Well, this probably won't be Mitchell's answer, but to me it's obvious that an uploaded human brain is less than 50 years away (if we avoid civilization-breaking catastrophes), and modifications and speedups will follow. That's a different path to AI than an engineered seed intelligence (and I think it reasonably likely that some other approach will succeed before uploading gets there), but it serves as an upper bound on how long I'd expect to wait for Strong AI.