It’s when the umpire hears it and not when s/he signals it.
10.5 END OF QUARTER
(a) The end of a quarter occurs when any field Umpire or emergency Umpire first hears the siren sounded by the Timekeepers to signal the end of a quarter.
The field Umpire shall acknowledge the siren and bring play to an end by blowing a whistle and holding both arms above their head.
(b) For the avoidance of doubt, if immediately before hearing the siren to end a quarter, a field Umpire is of the opinion that a Player should be awarded a Free Kick or a Mark, the field Umpire shall signal that play has come to an end and then award the Free Kick or Mark to the Player. A Free Kick will not be awarded where the football has been kicked and, after the field Umpire has heard the siren, lands Out of Bounds On the Full.