From what I get its 2.5-3 seconds in that video. You should be able to replicate it if you smash the button. On the other hand he could be using a macro to make it easier.
You could go into custom games, start a game by yourself and time yourself how long it takes you to empty an extended mag.
Decided to test how fast I can shoot the 1011-12 pistol. Its about 2.something seconds to unload the pistol with extended mag, no macros:

