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 don’t think it’s going to be possible to give an absolute timeframe here as the answer is probably that it depends.
I lean toward booking early as that generally seems to be a better idea than the alternative.