4 hours for me like clockwork after a few drinks before bed.
Almost always after a night of heavy drinking I wake from anywhere between 5am - 8am and have a rough time getting back to sleep for at least a few hours. Knowing my drinking patterns, those times would tend to correlate to about 4 hours earlier passing out.
From what I recall, alcohol binds to GABA receptors, GABA builds up in the body, eventually metabolizes to glutamate which takes a few hours depending on how much you drink, but seems to be around half a night's sleep. The glutamate wakes you up and gives you energy, hence the strange "feeling great after a night of drinking on no sleep" thing.