I book a lot of flights at my employers expense, and the general guideline is to TRY to book two to three weeks in advance. I don’t check the price myself, so I can’t verify if it holds true, but the thinking is that it’s on average cheaper - After the “we can’t be bothered planning this flightuntil we know it’s going to be profitable”-phase, but before the plane is almost full so they can start squeezing the passenger wallets.