Hi,
70,000 miles or 7 years, is right for the Izuzu engine. Don't forget that if you are stop/start driving etc, this will shorten the belts life. If in doubt about the state of the belt; change it. It's an easy job but it will save you a trashed engine, and don't forget to do the pulleys and tensioner too. Very often you can't see how perished the belt is until you take it off and bend it back, then you realise how close it is to snapping.
Cheers
Ken Chapman
April 2007