If you trying to ask about specific routes, like SIN-KUL then yes you can get cheap tickets a couple of hours before... but only if the flights are rather empty, never SIN-HKG have ai seen cheap.
If you talking about real long haul, like SIN-MEL i have never seen cheap last minute deals however i have seen the cheap classes become available, so to say you wont pay any less that the normal cheapest advance booking but youmay just pay the same... but if a flight is full... well forget it !
I have booked QF & BA so last minute a few times that by the time i had got to the airport they had not completed thie ticketing. (SIN-<somewhere>-MEL). Even once handed my card over at the ticket counter at the end of the check-in to get my ticket

but i had already purchased on-line so i knew the price was set.
E