Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
Toyota does not have a service interval for replacing the timing chain. On these vehicles, replacement of the chain is done on a "need to" basis, which is usually few and far between. These vehciles very rarely have issues with the timing chain.
To replace the timing chain, the labor time guide calls for 4.1 hours. With A/C add .2 hours. To replace the chain tensioner, .6 hours.
When you say "gears" are you refering to the camshaft gears? This takes 3.9 hours.