As soon as the dog is dry put it on him
How Long Do I Wait After My Dog's Bath To Apply Advantage?
Make sure that your dog is completely dry and then you can apply it.
The spots where you put the advantage will be rather greasy looking for a few days. So I would recommend applying the Advantage then waiting a few days (at least 48 hours) and then give the dog a bath. Advantage is absorbed through the skin and the information sheet in the box states that Advantage will still be effective after shampooing and swimming.