Everything PPs have said is true, BUT we've been as late as 45 minutes (obviously not planned and way out if our control) and been seated. Again if you're truly apologestic and not flagrently rude, AND if you truly were late for kind of an excusable reason, they would "probably" seat you. Over the years this has happened to us at least 3 different times, one time was TOTALLY my own fault ( I messed up the times) They were kind enough to seat us anyway! I'm not saying to plan on it, but things happen, life gets crazy (even at Disney) and CMs are patient and genuinely kind.
I also think it depends on the restaurant, how busy/popular it is and how backed up they are.