I think I remember it being one melee attack to knock them down. Then you've knocked the person down, you decide whether to execute melee style or you shoot him while he's down. Someone else correct me if I'm wrong, too vague, or there's more details.
Yes, I remember hearing that SD created a bit of a mindgame with the melee attacks. If you knock someone down, you can do a melee for an easy kill, but they are able to shoot at you while they're on the ground. So the basic idea is, if they are shooting, you shoot; if they try to get up, go for the melee kill.