I would wait 15 minutes before asking if everything was OK and if I had much longer to wait.
I used to have a dentist (some 11-12 years ago,) who was always ALWAYS behind. Like over half an hour to 40 minutes! Did my fecking head in! I would be the first appointment sometimes (after lunch at 2pm,) and he didn't call me in til 2.15-2.20pm. Sometimes he didn't come back from lunch til 2.05 to 2.10pm. Then it would be another 10 minutes before he called me in. So he was already 15-20 minutes behind and he was only on the first appointment of the afternoon. A few times, the 2.15pm appointment had come in and I was still sitting there. And I even remember the 2.30pm appointment rocking up too when I was still sitting there at 2.22pm!
There was never any reason for the lateness, and quite often when my appointment was mid morning (like 11am,) he would be 45 minutes late. I kept asking what's going on, and there was ALWAYS an excuse. Pissed me right off it did. I was made to feel like I should be grateful to have an NHS dentist. 
I now go to another one near the place I live now, and have been going there 9-10 years. She is never EVER more than 5-7 minutes late. She is usually on time or just 1-3 minutes late.
My good friend who has a chronic illness has an appointment with a consultant every 4-5 months, and he always, without fail, calls her in 45 to 50 minutes after her appointment time. Sometimes he is an hour late. The people/patients before her, go in to see him, and spend some 25 minutes in there, but she is in and out in 5 minutes. She has said she is getting annoyed by it, but the receptionist said 'some of the patients are more complex.'
It can't be this every single time. He is always ALWAYS late. Like, close to an hour late. Not just 10-15 minutes late.
It's so fucking rude. Like their time is more important than yours. As has been said, if you are more than 10 minutes late, you lose your appointment. Such a superiority complex some people have!