“The average human eye blinks at a speed (between) three-tenths and four-tenths of a second. So if you are the batter and you blink at the point of Chapman’s release, the ball will pass you before you open your eyes again.”
Matt Bynum of Hillerich and Bradsby quoted by Paul Daugherty – SI.com.
Estimated time from Chapman’s hand until the ball crosses the plate at 104 mph — 0.36 seconds.