how to know when it is the time to change the timing belt ?

If your vehicle has greater than 100k miles on it, and you have no idea or no records on when your timing belt was changed, it is probably time to do it, as you are on borrowed time. Unless you can see when it was changed by records or by the dealership where you bought it, I would say get it changed.

Usually a sticker inside the engine bay with the mileage when it was done then as above around 100k

When you hear a snap followed by 8 pings it is too late.