Wow, how did you manage to drive so little in 18 years! Haha. Anyway, that is a really good question. My impression from this forum is that timing belts are still in pretty good shape after 100,000 miles, but almost everyone is driving at least 10,000 miles yearly. A rubber belt is going to be a lot like rubber sidewalls on a tire. It degrades over time just like tire sidewalls show "dry rot" or whatever they call it, whether or not the tires have gone a lot of miles. Tire sidewalls are often in bad shape after only 6 or 7 years. My understanding is that an interference engine like the 3.3 L V6 can be badly damaged if the belt breaks or slips, so I don't believe that I would try to get the prescribed 90,000 miles out of it. Oxygen and ozone in the air slowly breaks the rubber down, "dries" it out and cracks it. I don't think the manufacturer of the belt is going to say it's good for 20 years, but you could look and see if they make a recommendation. I think someone on this site said that Aisin was the original equipment manufacturer for the belt. Unless there is some way of inspecting the belt much easier than the repair itself, I think I would replace the belt pretty soon before 90,000 miles. Should probably do the water pump at the same time.