Not more than a paragraph or two - unless they type like 120 words per minute.
OCR is getting pretty good, and easy to use. I would NEVER OCR text without looking at the result carefully, though.
The version we used, was a pain, required manual intervention. To guide it when it was confused and then to correct the errors. Accuracy was extremely important to us. The tool that we used was commercially available.
Sometimes the source was so rough that the OCR would guess wrong and do a terrible job.
Interesting... the human brain can interpolate based on sentence grammar and conceptually when a software program cannot. A computer cannot deal in concepts and ideas. This is AI and pattern recognition both at the shape matching level and conceptually.
We just ended up giving it to our documentation person who transcribed it and she was good.