Er, I hadn't thought about that. Like a week at most? I mean they could be infected for a couple of days before showing symptoms and then they become rabid. Maybe their immune system deteriorates or they become hemophiliac, so any fights between each other help pick off a bunch.
Also, how soon do you want to encounter multiple infected? I kinda had something in mind for Paige's boss who was left alone in the pet store...with a large snake.