When should I get the timing belt on my 2000 Toyota Corolla changed?

Toyota recommends changing the timing belt every 85k which is a god idea in that Toyota builds interference motors. If it hasn't broken it is a good preventative maintenance. Also it is a good idea to change the water pump while you are in there as this is a task to get to when replacing it and can cost you just as much as replacing the timing belt.

easiest way to change a timing belt is to
first slightly slacken off the belt tensioner
carefully cut the old belt in half along its entire length while it is still on the engine.
remove the outer half only.
place the new belt on the pulleys up against the inner half of the old belt.
cut and remove the inner half of the old belt.
push the new belt fully into place.
re tension the belt as specified.

one of the sensors in the belt latch is bad

The 2000 Toyota Corolla will have a timing chain. Earlier models have the Belt. The chain is pretty much maintenance free. When it is time to replace it you will usually start to hear a rattle where the chain is losing tension and is vibrating against the chain guide.

80000miles or 6years

your corolla has a timing belt. try a Haynes manual. you can find them at your local parts store. there are too many steps to relay here.

timing belt, tensioner, idler pullies, possibly water pump if the bearing is worn or the seal is leaking.

belt, it should be replaced every 90,000 miles

it has a belt

Your 1999 corolla has a timing chain not a belt. No need to replace!