In here it’s discussed, no real interval , just cheaper to change the chains and guides , then replace bent valves or damaged pistons , or complete engine.
after 125,000 miles you have to ask yourself, do you feel lucky? Well do you?

At that mileage, if you take it apart to check, continue and change the timing and balance shaft chains.

https://www.chevyhhr.net/forums/how-...in-kits-60394/