According to Frontline you can bathe your dog after it has been applied as long as you wait 24 hours after applying. This gives it time to spread and dry after that it stays on even with bathing and shampooing.
Make sure when applying it that you get as much on the skin as you can, even if this means applying it in lots of little spots on the back of the neck. Usually, if you apply it all in one spot too much of it gets on the hair shafts and won't spread properly.
(vet nurse)