Yes, they will continue to harden for anywhere up to a year. However, for most practical purposes, you only need to wait a couple of weeks.
If they are sized after they have already hardened, the sizing process will remove some of the hardening.
These are generalities, The exact composition of the alloy, the temperature change (whether through dropping the boolits from the mould or through heat treating) and the storage temperature will all affect both hardness and the amount of time the hardening process takes. I've done some long term tests where certain alloy mixtures continue to harden for 18 months. None of these variables affect the hardness to a large degree, and waiting a few weeks is sufficient.
And, I have to add, I don't use a lot of heat treated boolits, I just like to experiment. I have a large supply of linotype and monotype, which is my favored method of adding hardness.