What is a Light Year?

Do you mean about 12 miles per minute? I think you do. About 5 seconds per mile is about right.

I think the standard answer is about 761 miles per hour... but it is far from constant. On a hot day it's faster, and on a cold day, it's slower. It slows down quite a bit with altitude, too, but mach 1 remains the speed limit.

For all practical purposes, the speed of light may as well be considered a constant... but the things that effect the speed of sound are calculated constantly by people in the business of flying... and especially those in the business of flying close to the speed of sound.

The speed of sound in the atmosphere at sea level is roughly 1100 feet per second, so Bill is correct that this is about one mile in five seconds. For the distance measurement challenged, that's about one kilometer in three seconds.