Third Date Syndrome...

How long can we pretend to be something we're not?
Turns out Mr Darcy with the great manners, was in fact a heavy smoker (hid that well for first few dates) and also seems to be an entirely different kind of guy when intoxicated (perhaps an anti-Mark Darcy almost). Date two superb, looking forward to date three, when un-announced and overly-intoxicated, turns up on doorstep, crashed a night out with mates, only to act the fool, disappear at every new bar, continue to drink heavily before disappearing at the final bar...
Third date syndrome? or just third date douche bag?
I'm quite convinced that we all on some level put our best foot forward in the initial dates. No one wants to see the 'real you' on date 1, but as you become more comfortable, it's only natural to think less and be more yourself. I'm not advocating that you pretend to be someone your not, but telling someone you only have one quick smoke when you drink, when its a blatant lie, is never going to work out.
But how soon do you drop the guard? And how much of a guard do we expect the other half to have?