How fast does a zebra have to run until it looks gray?

A zebra can run between 30 and 35 miles per hour. Or about 500 feet per second. (35 times 5280 feet divided by 60 minutes and divided by 60 seconds)

Depending on how close you were to it when it went by, it could be a gray blur. But even at fifty yards in front of you, your eyes would track and follow, so that the zebra would be "still" within your field of focus. And thus would not be gray.

Bear in mind that white and black frames have to be flickering at a rate of 60 frames per second before starting to gray out. And that zebra is moving faster than that. But the Zebra's isn't winking in and out, your eyes are attracted to motion first, and you automatically track it. Thus letting the image resolve as a "still" against a blurred background.