It depends but often cheapest prices IME are;

1) as the flight is released (e.g a year in advance or more)

or

2) about 5 weeks out.

As you say it depends but in my case when I was booking a ticket for my last vacation I noticed prices getting much cheaper every day nearer to the flight itself (500 -> 200).

I waited till the second to last day and the price was 240, one day later it hit 200. (definitely a gamble, the flight seemed maxed out when I was boarding).

Sometimes, the day before or the day of the flight too.

If they haven’t been able to sell the seat yet, the airline is in a big hurry.

If they have sold almost all the seats, the last 1-2 will often be crazy expensive though.

High risk.