As per all my answers here, this is based on what I'd do. That may or may not be useful to actual humans with social skills :-)
This is one situation where it's actually useful to be diabetic. You can just claim that, for medical reasons, you need to eat at a specific time or risk going into a hypoglycemic coma and dying :-)
For one-on-ones, the whole point of the meal is to share it with the other person, so I'd wait as long as necessary, perhaps ordering bread or small entrees to keep yourself going. Then, when they arrive, make it clear you just needed some snacky stuff and then get into ordering the real meal.
For a larger group, the point is to share time with not the entire group, since that's difficult enough even if they're all present. Rather, it's to share time with various people or sub-groups within the group, as needed or desired.
So I'd tend wait a while for everyone to arrive (say about 15 minutes tops) and then order. I don't consider the cutoff time to be rude since:
- someone almost certainly set a specific time for the get-together, which was known by everyone;
- the people actually running late are being rude (if intentional);
- or they should not mind if you order (if not intentional).
In any case, there's no requirement that you go your meal like a starved pack animal, you can adjust your eating speed so that hopefully you won't be finished before they arrive. Even if everyone started together, there would be situations where some would finish first (a friend of mine almost choked on a chicken bone when young and therefore eats notoriously slow, so we were often waiting around for him to finish).
If, after your small wait and slower eating speed, they still arrive late, maybe next time they'll plan better :-)