Nissan states when to replace my timing belt. They should be very clear, it's in their best interest to keep you informed about proper maintence over the life of the car. What is their idea of life of the car? In my opinion, I'd estimate 150,000 miles or about 15 years for a early '90s car. Maybe up to 200,000 miles and 20 years today. Used to be if you made it to 100,000 and 10 the manufacturers felt they did their job, and you were on your own. Now it is a lot longer.