I generally start looking maybe 2 weeks in advance talking to reservations. They'll at least tell you if it's full, so you know which ones to forget about. But keep checking. A flight with seats 2 weeks in advance still has 2 weeks to fill up

And even then, you might have a chance. When I nonrevved on IB MAD-SVQ, they told me that the first flight was looking a bit full, but the next two were better. Well, I was in MAD anyway, so I had no reason not to try. Talked to their customer service desk while I was waiting, they said I had a decent chance. Got on that flight, no big deal.