How does anyone know when to have their timing chains changed besides guessing on the need to have it done based on mileage travelled like 100,000kms, 200,000kms, 250,000kms, 300,000kms ?????

It would be risky to wait until it breaks! Then what? There will be lots of damage costing tons of $$$. So is it true that you have to open the top before examining the chain and guides to decide? Then you will also have to change a new gasket.