Primarily ..... When you need it and have the funds needed to meet your goals.
Secondary considerations would Include:
1. When a new generation product is released, prices on previous generations go in the toilet... part of that will be right away ... a 2nd drop will occur when new gen stuff is available in quantity.... example : Z270 / Kaby lake replacing Z170 / Skylake
2. When a new top dog takes the crown.... Example: When the 780 Ti was announced, the price of the 780 dropped $160; few weeks later it dropped again. We see this starting now with the 1080 Ti ... once the AIB cards drop (2 weeks ?) and are available in quantity we shud see a tumble.
3. New competition arrives ... to hit the ground running, a manufacture will only introduce a competitive product when the can show a price / performance advantage (real or imagined) of the status quoe ... competitors will respond with their own price cuts ... again we see the two stage drop ... 1st upon announcement, and 2nd when a) they arrive in sufficient numbers to have and impact and b) when the product fails to measure up to the pre-launch hype. We should see this occur soon when AMD introduces their new cards.