From day one (or maybe day two)
I was brought up with "role reversal". On my leg as PF I got to act as PIC, make the logbook and handle everything from communication with ground crew and decision making. My DFO saw it as Line Training "light".
Once I did my command training, I had already tried the role as PIC from my comfortable right seat.
The guys in the left seat who put some more effort in to this was always keen on discussing matters during cruise etc. Fantastic way to do it i think.