When do you need to replace them? I have a 2001 Toyota Sienna XLE. It runs fine. It has 60535 miles on it. The maintanance schedule recommends to replace them at 60000 miles. But they work fine. Do I really need to replace them?

Hmm.. they're probably still okay but 60,000 miles will have reduced their efficiency.
If it's a decent car, I personally would replace both the plugs and leads - when they are in good condition the engine will burn less fuel and run smoother.

Long-life plugs should be removed every two years, in order to prevent seizure. Then, they can be reinstalled. If they are left in too long, the softer threads in the aluminum head can be stripped upon removal. Remove plugs when the engine is "stone" cold. Follow car manufacturer's recommendation on whether to use antiseize compound or not. If antiseize compound is used, reduce torque 20% to prevent stripping threads. Personally, I would replace the plugs in your car with new ones.

It's a VVTi motor. They put iridium plugs in there and they last way past 60K. We recommend at 100k. What was said above may be true... but if it were my vehicle, I'd spend the money elsewhere.
BTW, the back 3 plugs can be a bear to get out of there.