at what mileage should you replace timing chain/belt

If you have a timing belt you replace it before it's recommended, for example every 5 years or 60,000 miles. The time and mileage for your particular vehicle will be in your owners manual. If you have a timing chain, they generally do not need to be replaced unless they are making excessive noise, stretched, or even completely failed. Most timing chains will not need to be replaced for the life of the engine if that engine was maintained properly.