There isn't any official rule, but usually they'll give you 10 or 15 minutes after your ADR time, but as the PP said, they do recommend that you show up about 10 or 15 minutes early. If you are late, they might put you on the walk up list or if it's very busy turn you away. We've seen both happen. At one very busy restaurant the CM told a couple they were 16 minutes late and they were given the option of being a walk up. At another restaurant which was running far behind schedule latecomers were told they could no longer be seated.