I was told it's every 60k/5 years by my dealer. That sounds expensive! It's normally £300 at a Lexus dealer, unless they are trying to con you!
The manufacturers established 60k as an interval for many engines since in experiments and other observations, the average time to failure was somewhere in the region of 60k, so they figured that's good enough. In some new engines, the interval can be as much as 80k.
However, the original LS400 engine was non-interference, meaning if the belt snaps you are not left with bent valves etc. Not sure if the newer models are the same. The belt will have become tired with age even though you might not have done a lot of mileage.
Either way, prevention is better than cure.