Lexus, like other manufacturers, plots the time-to-failure on the belts, and sets a point at which almost no belts fail as the interval. Of course many will go past that interval before failure - almost all of them in fact. That's no reason to ignore replacement intervals. Just because you miss an oil change or two doesn't mean the engine will blow up either. But we change oil regularly anyways.
There is no accurate or objective means of "inspecting" a belt. At the same time the belt in my 98 GS was obviously past it's prime at 100k kilometers, or about 62k miles. I changed it because the 7 year limit had been reached, and I'm glad I did.
I have changed hundreds of belts in my time. I have replaced belts that looked good, and many that looked terrible.
The Vvti engines are interference. Have one of them fail, and you'll be crying for weeks.....
Thanks for this thread, it reminded me I need to do my belt/pump, which has been easy to ignore because I have such unusally low mileage for a '98 (under 34K). But last time I had a 5K service (about 3 mos ago), my mechanic, who I do trust, suggested I change TB/WP due to the time that has elapsed. He did a visual inspection, said it LOOKED OK from what he could see, but as stated here, you can't trust a visual inspection. Techinically speaking, I'm way overdue.