...No, I think it actually depends on an ever-fluctuating market. There is no magic number. If you want to buy low, check prices regularly.
Theres no way to tell though I suspect they will slowly start to drop from as soon as the Crs announced but the total ammount they will drop has too many veriables and is ungessable.
If you want to be a market player, study the market well, like ghelas told. There is no set time for a price-crash, it can happen to weak cards that were hyped up by cr specs faster, and good cards may not even need to come down in price.