Shortest period of time ever recorded is the time between when a fireant gets on you and the time it starts biting/stinging.
That's no joke. A wasp might land on you, walk around a bit and fly away without ever stinging but fire ants start attacking as soon as they touch your skin.
Just this past Monday I wasn't watching where I was going and set some gear down by a fire ant fill. When I went to pick it up, ZAP! ZAP! ZAP! Like electric shocks, I started getting stung. Most of the bites went away fairly quickly but one on my finger has turned into a nasty, pus-leaking zit.