If the storm is overhead the thunder and lightning are seen to occur at the same time, otherwise there is a delay between the lightning flash and the thunder clap, due to the difference between the speed of light and the speed of sound. Because light travels so quickly, it's safe to say that the light from the lightning can be seen instantaneously. The sound from the thunder still has to travel through the air to reach us. As sound only travels at 761 miles an hour, it takes approximately five seconds to travel a mile. So each 5 second delay represents about a 1 mile distance from the storm.
Thunder storms die out once the charge between the ground and the cloud is equalled.