The government can start an extraction any time it likes regardless of the vote result. But in all practical situations they're not going to go against public opinion nor ask again within a generation. (Barring something dramatic happening like the rest of the EU breaking up, or there being a UKIP government elected.)
oG has got it
we can invoke article 50 ( or whatever ) whenever we ( our masters ) like
There are certain questions as others have ointed out
where you / one goes on asking it
shall we do X - no
shall we do X no etc
until
shall we do X yes - ah yes well we have a definitive answer and there is now no point in going over old ground .... the asnwer is settled for a generation
asking about the common market in 1973 was one
and scots independence seems set to be another