Last night, we had rain, lightning and thunder. I woke up a couple of times to listen and count how long between a bolt of lightning and the thunderclap.
Sound travels 1 kilometer in roughly 3 seconds and 1 mile in roughly 5 seconds. When you see the flash of a lightning bolt, you can start counting seconds and then divide to see how far away the lightning struck. If it takes 10 seconds for the thunder to roll in, the lightning struck about 2 miles or 3 kilometers away.
I counted from 15-20 seconds, so the lightning struck about 5 to 7 kilometers or 3-4 miles.