How do you know when the timing change go? Thank you.

Technically, timing chains are designed and built to last the entire life of the engine. So in theory, you should replace it only when it's broken. However, if the chain breaks while your engine is running, it could wreak considerable havoc, so waiting for a break isn't the best practical strategy. Before a timing chain breaks there are signs of wear, and it isn't hard to check for them. If your Jeep Cherokee is showing some of the signs, it is time to replace the timing chain.
Timing Chain Function

Pull the distributor cap and turn the engine over and see if the rotor button is turning,if it is,then the timing chain is not broken,but it still can be out of time,the timing chain might have jumped and still be attached.this will take a little time to diagnose,align the crank to top dead center and see if the rotor button is pointing exactly where the number one spark plug wire is supposed to go on the distributor cap,if it is not then the timing has jumped.