Logic says when the card has lived 2/3'rds of the new bloods stage it is about as cheap as it gets (if the card is worth playing).
There is large supply and the fast demand has been quenched = price is low. Later when card is about to leave newbloods investors are stocking it because it's supply will be lower later - so if they plan to buy lots then it has to happen while there is still supply.
45 cards in the newbloods cycle currently, 4 cards per half-month = around 11 cycles - 5.5months - to cycle newbloods completely.
So roughly when card is 7 cycles old - or 3-4months - it's price is at bottom most likely. This is only general rule of thumb.