I don't know the exact numbers they use for their statistics (I think it used to be 1 minute).
If a train is delayed by 10 minutes you can ask for a ticket to take to work to show your boss that being late was not your fault.
Don't get me wrong, the punctuality is much better than most European countries I have lived in and quite a lot of the time the are perfectly on time, but the west has raised the Japanese public transport system to near mythical status.