rule of thumb would be the sooner the cheaper, but how many weeks or months in advance?
For my particular flight there are only three companies and it seems flights are cheaper between Tuesdays and Thursdays. However, if you want to book a flight 2 months from now, it’s considerably cheaper.
Have you found the sweet spot?
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.