I always wait at least three days...but you will see that in 1 day it will dry and your baby won;t even look greasy

BUT i went to their website and here's what it says:
8. How long after application can my pet be bathed or go swimming?
FRONTLINE Brand Products remain effective for 30 days, even if a pet swims or is bathed. After application, keep the dog or cat from getting wet until the application area appears dry, usually 24 hours. If a FRONTLINE Brand Product is to be applied after a bath, make sure the pet is completely dry before application.