Up to the restaurant - no hard and fast rule.
We watched a family walk up to Steakhouse 71 who were 1.5 hours late for breakfast (and they looked like they had a rough morning!). The host was surprised and had them repeat their reservation time, to make sure she heard them correctly, but they were seated not long after us.
Things happen, and I think most people are as accommodating as possible. Just know it's probably a gamble after 15-20 minutes, esp in really popular/booked restaurants.