Of my last 10 flights 90% left on time as per the 15 min rule from the OP, so it would appear I have been very lucky compared to others. This included 4 long haul all of which left on time and 6 short haul 5 of which left on time, with the 6th being cancelled the day before. I was put on a later flight that day, 11 hours later as that was the next available flight.