what is average mileage to replace timing chain

The 4.6 DOHC engine romeo based is a very fickle mistress. Typically to answer your question it would be shortly after 100000 miles. This can be extended or reduced based off of the usage of the vehicle. If this something you plan to do your the FORD tools for timing are very helpful but not required. Most people wait until they hear the notorious ticking then replace the tensioners, guides and chains. This can be done with the engine in the car but the radiator must be removed. This can be done in a weekend if you have all the tools gaskets and thorough instructions. It is easier with a second set of hands but not required. Email me if you have any questions I've done this on 4.6 based engines many times in my driveway, remember you basically have a cobra mustang engine under there.