It depends on how busy the restaurant is, are they on schedule or running behind, how much before your reservation you arrive. In other words, there's no way of knowing what will be happening that particular day. You might be able to check in or you might be asked to come back closer to your res time. If your reservation is 10:45, Disney recommends that you arrive 15 minutes early so 10:20 might be okay, but if you decide to show up at 10:00 be prepared to be asked to return later.