So, I know that the prevailing theory as to when Lea was infected by Nemesis is when she was given the dagger in Grave Peril. I'm currently on a re-read and just noticed some very convincing proof that this is exactly what happened.
As a refresher, shortly after Bianca's party, Lea confronts Harry and captures him in the Nevernever. Harry poisons himself and Michael makes it known to Lea that he has an antidote. Lea then immediately demands b that Michael give it to her. Lea is one of the top ranking Sidhe in the winter court. Accepting a gift would incur debt and could v have disastrous consequences for her, which she is very well aware of. So much so that she brought an offering to offset Bianca's gift of the knife at the party. It's completely out of character and it's the first time we see her after she gets the knife.
It seems to me that the effects were rather immediate. Anyone got any thoughts or observations to add?