The visibility range of a signal mirror largely depends on weather conditions, the size of the mirror, and the skill of the user. On a clear day, a standard-sized signal mirror (around 2 to 3 inches in diameter) can be seen from up to 10 miles away, and sometimes even further if conditions are ideal. Larger mirrors can increase this range, but the key factor is the intensity of the sunlight and the lack of obstructions like clouds or landscape features.