The factory recommends every 90,000 miles or 9 years whichever comes first. I say between 90,000 - 120,000 miles is ok. As for years it really depends on climate and driving conditions. I bought a 2005 LS430 in 2019 with 94,000 miles. I replaced the belt soon thereafter and it showed no cracks and minimal wear after 94,000 miles and 14 years. In 2017 I bought the 2000 LX470 with 46,000 miles and the belt had no cracks either and probably could have gone longer, but I also replaced it. Peace of mind prevailed in both instances.
I would say every 10 years is a good number if you haven't reached the mileage by that time. Remember that its not only the timing belt, its also the water pump and coolant flush. The idler bearing and tensioner can actually go for much longer.