I think that they will wait until the last possible minute, e.g., 10 minutes before departure, by which time all passengers were supposed to have arrived at the gate, and fill the plane with stand-bys.

I once had the unfortunate experience of showing up just under 10 minutes before departure and having them bump a passenger to whom they had given my seat a couple of minutes earlier. I guess the plane was totally full. (This was the kind of accommodation that makes United 1K status immensely valuable.)