They don't do repairs of iPhones while you wait, appointment or not. If they determine your phone has a defect that's covered by the warranty, they're going to swap you out with a replacement phone (again, regardless of if you have an appointment or not).
At the store I worked in a for a few years, if you didn't have an appointment, you may end up waiting up to an hour-and-a-half before they can squeeze you in. Even if you come in at a not busy time (say, an hour before closing), thinking "hey, it won't be busy then, they'll squeeze me in quickly without an appointment", keep in mind that the number of employees working at that time of day is also less, so it's usually no easier for them to squeeze you in then.