Longest
I remember one Friday there was a thunderstorm that lasted over 12 overs more or less, it started around 3pm and lasted all through the afternoon, all through the night and into the early hours. It happened from what was not a particularly threatening sky either, it was a pale white overcast sky.
Shortest
I Greece on a very warm evening. Had a few flashes of lightning, a brief shower that lasted a few minutes and then went. The people eating outside did not even budge.