[theroyalburgh (OP)]

Lightning only takes a few milliseconds to reach the ground and return to the cloud. Thunder is the noise of the air molecules returning to the space created by the strike. So if the air returns back to its former space in a few milliseconds, why does the noise of thunder last so long (5-10 seconds)?

[daveshorts]

It depends how far you are from the lightning. If you are very close the sound is very short, if you are a long way away it tends to be much more drawn out.

This is because the sound can get refracted and reflected off different objects and layers of air, so the thunder sound can have travelled several different distances to get to you. This means that you keep hearing the bang again and again all blurred over one another, so you get a long rumble rather than just one bang.

[lyner]

Dispersion of a short, high energy impulse into a longer lasting disturbance.  Different  frequencies have  different speeds. Also, there are echoes from surrounding hills and clouds (the change of speed in clear air and  the air within a cloud will cause reflections at the boundary).  You get a whole series of delayed signals which becomes a formless rumble. The further you are away, you will notice, the less of a 'crack' and the more of a rumble is heard. The initial impulse is extremely intense and there is a lot of energy about.
Daveshorts got there just before me! Another example of dispersion, perhaps.