My best shot with a signal mirror was eleven + miles, across a hazy, polluted Tucson sky. The helo pilot knew where we were before he even lifted off. We used them extensively and effectively, commonly for locating teams on the other side of a canyon, etc. I can recall at least one occasion where a party in trouble attracted needed attention through use of a signal mirror. They thought they were sending Morse code, while observers saw only disjointed mirror flashes. It was still enough to arouse curiosity.

A couple of points - it is possible to overuse them. Do not keep flashing the aircraft once they have a bead on you. It will be uncomfortable for the pilot.

Signal mirrors are great for putting light in dark places like mine shafts, rock shelters, and cave mouths. Learned this trick from mine safety officials, whose device for cave shafts was the mirror off of a standard size medicine cabinet - very illuminating!