"Speed of Light Drill"

The sketch below shows the basic elements used by A. Michelson to measure the speed of light in the 1920's. A pulse of light hits the rotating 8-sided mirror and takes off on a round trip to a flat mirror some distance off. The pulse will be received by the detector if the rotating mirror is in the right orientation when the pulse returns. A moderate price power drill can turn at about 1200 RPM . If such a drill is used to turn the mirror, how far away does the flat mirror have to be for a pulse to be observed?

Hint: Assume the distance to the mirror (L) is large, so you can approximate the round trip as being exactly 2L.