Interval is 100k miles or 10 years.
Just had the belt changed on my series I RX300 after 105k miles. Asked for the old belt back, thought the garage hadn't changed it and given me the new one. Not a mark or crack on it, would easily last longer. On US forums people have gone 130k miles+ on an original belt (not that I advise this).
It's only the IS200 that has an issue with the cambelt and needs to be changed more often.
Why not chains? They can be noisy, are less efficient, water pump normally cannot be driven from them because if the pump locks up the chain will snap and do a lot of damage.