Timing chains are designed to normally last the life of the engine and are replaced during a complete engine rebuild.
The Ford Mustang does not have a timing belt, it has a timing chain that should last the life of the engine and requires no maintenance.
Mustang's have timing chains not belts and do not have interference engines. Change the timing chain when it brakes or stretches to the point it throws off your valve timing.
I'm not a mechanic / technician but as far as I know there is no scheduled timing CHAIN change on an engine like there is on a timing BELT
Timing Chain.
The 3.8 liter V6 engine has a timing CHAIN
The Mustang V6 does not have a timing belt it has a timing chain which normally will last the life of the engine.
The 3.8 liter V6 engine in a 1996 Ford Mustang has a timing CHAIN
Timing chain
The V6 engine in a 2004 Ford Mustang has a timing CHAIN
The 2003 Ford Mustang does no have a timing belt, it has a chain which requires no maintenance. Replace the water pump when it starts leaking. It should last 100,000 miles or more.
The 3.8 liter V6 engine has a timing CHAIN
The 3.8 litre V6 engine in a 1999 Ford Mustang has a timing CHAIN