In my experience its not as predictable as that-
I have found the prices to be random. Can’t see any regular pattern.
From experience I know what a decent price is so if I see it I book it, if not I wait and see.
The best prices I have had so far are early bookings to the canaries - but that’s not always the case.
This is for p&o and Marella.
Saga is much more predictable - earlier the better but also a price guarantee obc credit if the price you paid is reduced.
don't know about other lines.