Ok based on some flights I have done and data gathered:

T-12 bidding upgrades
T-6 Black/BS upgrade clears until 3 seats are left.
T-2 All upgrades clear including Plat/Gold

i have not seen too many bidding upgrades on long haul flights. Its difficult to tell as it could be a confirmed passenger changing dates or someone purchasing last-minute J.

You cant bid after T-12 so this reinforces my point hat if there are seats after T-12, those seats will go to elite upgrades unless someone buys J directly or changes flights with confirmed J.