About as long as it takes to go to your brain - once things are in your blood they're in your milk as a general rule, especially if they are fairly small molecules like caffeine. The big thing is the rate that it goes out - you "process" caffeine fully in a few hours, a newborn can take a day or more to do the same. As they get bigger they should process faster - but my DS was well over 2 before I could have a second tea without it affecting his sleep.