The timing debate rages on at my outfit. As others have mentioned - it is a visual manoeuvre with the final turn to be commended at when the a/c is at 45 degrees to the final approach or runway track.
On Airbus, another way of helping with this is a fix info line as a radial from the landing runway threshold, 45 degrees off. When the aircraft crosses the line, it's time turn. Particularly helpful if the runway lies on the other side to which the PF is sat, and PM happens to be busy (copy active instead of activate sec, anyone?)
Just another guide, and helps with overa SA.