This is copied from their website:
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. FRONTLINE | Helpful resources | FAQs