According to the manufacturer, it is good for 30 days. My vet says I can use it at 45 day intervals. Dogs should not swim for 3 days after application. Also, if the dog is going to be touch a lot, the 3 days should be observed to prevent the stuff from getting on skin while still wet. So the short answer is 3 days minimum before you leave.
PS: I don't believe that I just answered a question from Labby. It has always been the other way.