There are (at least) 3 significant dates in play here:
A. The date the itinerary is first established (Booking Date);B. The date of the first flight in the itinerary on Booking Date (First Booked Flight);&C. The date of the first flight in the itinerary after the last itinerary change has been made (First Actual Flight).
In regards to A, I'm being told on the phone by Qantas that 1 year from A is when First Leg must commence. I don't think this is correct at all. Anyone able to shed light?
I think that C is a bit of a stretch as it means that you can perhaps have infinite validity (if you want to keep paying the change fee). Those that got away with this I would say they just got lucky.