We changed at 90K miles.

We changed at 90k miles according to the manual.  But I believe it's a very conservative estimate and many can go 150k miles without problem at all.  One of my friend has over 200k miles and still working fine.  Also, Sienna has non-interference engine so there is not much extra cost if it breaks except towing charges and inconvenience.  But I am a conservative owner and want to be on safe side, so we changed at 90k plus water pump, driving belts and oil seals if you expect to keep the car for a while.