For my GSF the spark plugs(Denso Iridium Long Life) should be replaced at 100K kms... But, I think I will NOT do it at that mileage...
Why???
Lexus does it just because of the recent very strict pollution regulation. Without new spark plugs your Lexus should still do at least 95% good which is still much better than those problematic 5 year old European or American cars!!!
My two Nissans only need to replace spark plugs(also Denso Iridium Long Life) at 160K kms recommended by Nissan. However, I changed the spark plugs for son's 2007 G35S at 100K kms... Yes. I got slightly better idle and a little better pickup after the change. But, it's totally fine if you don't change the spark plugs...
So, this time I didn't change the spark plugs for my wife's 2010 M35X... 7 years ago I did 0-96 test on that Nissan and I got 5.82... Guess what... Last year I did an exactly same test on that Nissan again with nearly 100K kms(at least 95% in short distance city traffic) and the result was 5.83!!!
PS: Yes. If you really picky and care about that 0.01 difference, then you should change the spark plugs at 100K kms...