When do you change timing belt in a 1997 mustang Gt?

There's no timing *belt* in a 1997 Mustang GT. It uses a timing *chain*. Timing chains generally last much longer than belts and rarely require replacement unless there's a significant problem (e.g., chain stretching, significant wear).

While there's no scheduled replacement interval like there is for a timing belt, it's advisable to inspect the chain and its components during major engine work or if you notice unusual noises coming from the engine (rattling, etc.). If problems are found, then replacement would be necessary. However, it's not a regularly scheduled maintenance item like a timing belt would be.