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?
People keep saying 3 weeks which is odd to me. When I book a flight that close, I feel like I tend to end up paying more and/or having a very undesirable flight time or number of stops. Several months out is what I try to do for the cheapest flight at the most desirable time/number of stops.