This is a 'how long is a piece of string' question. Some airlines give an allowance, one of ours calls anything up to 3 minutes late as on time. It can also be very busy with ramp congestion at times
Generally it is a quick conversation between Captain and Despatcher about what really went wrong. If the Captain is happy that everything possible was being done and it just didn't work then it's on time, if not then you have to start looking for a reason, but anything upto 5 minutes seems to be the norm.