"I'll be a couple of minutes late" translates as anything up to 15 minutes after the agreed time. "I'll be 15 minutes late" translates as anything up to half an hour. "I'm running very late" means it might be time to look for more punctual friends.
We have what's called "Vicky time" in our office. On time to her means 15 minutes-30 minutes late. She's also constantly on her phone or blackberry and seriously addicted to it. I adore the woman--she's hysterical and awesome at what she does. But sometimes, I want to rip the cell phone/blackberry out of her hands and throw it into the bottom of Lake Washington.
~Heathyr
recommended if only because Heathyr's comments made me laugh