How often do the spark plugs need to be changed on a 2004 Toyota Sienna?

That depends on the electronic ignition system. Read the manufacturer's recommendations in the owners manual. Most GM vehicles are now suggesting 100,000 miles on a set of plugs.

I called Toyota Canada and they told me they are lifetime spark plugs which never need to be changed. However, no matter what kind of plugs they put in the engine from day one are never lifetime. I have 110,000kms on my van right now and I am going to change them. I am starting to see a decrease in fuel milage so I guess that's when you should think about changing them out. Might not be the problem but never hurts to change them out. I am going to go with a really high end spark plug the NGK Laser Iridium plug which will last me over 100,000km's.