I just got off the phone with a company that only repairs Lexi in the Dallas, Texas area, specifically Plano. They were all Lexus mechanics before they started this company. He told me Lexus timing belts are made of Kevlar and that they rarely, rarely break.
He feels that replacing just the belt as dealerships recommends simply rips off the consumer. Mr. Wright said that what would break is the water pump.
If you belong to alldatadiy.com, you will see the Chilton-like breakdown of what is involved in replacing the parts. The timing belt must be pulled to get to the water pump. So Mr. Wright replaces it (about an 55.00 - 80.00 part) when he replaces the water pump, 2 cam seals, crank seal, idler pulley and tensioner and the water pump thermostat and adds new, factory, silicon free antifreeze. He charges $1100.00 and gives me a loaner for the day.
He said he has about 16,000 customers and never recommends replacing the timing belt.
Alldata DIY is really great. I am in no way affiliated with this service. It is the closest I can get to Chiltons as there is NO repair manual out there that I can find. :)