When is tune-up

I've read on this site that most people change their spark plugs before the recommended 100k mile interval. What would you guys say is the average miles to put on a Malibu before changing the spark plugs and plug wires? My '03 Malibu has about 42,000 miles and I'm curious when others have replaced the plugs and wires.

I had to change the wires at 68K miles. One of them had completley deterioated. Probably spelled that wrong. I replaced the plugs a few hundred miles later. The plugs were ready to be changed.