Hello and welcome to the LOC mate.
The timing belt on your car should be replaced at 100,000 miles or 10 years according to the lexus warranty and care book page 14, but for piece of mind i'd personaly have it changed and get the belt tensioner done at the same time? There are a few topics on this on the loc site so if you use the search function you should be able to find out what other members have done hope this helps
Chris... :)
Gord you must be able to type faster than me mate :D