how long do the non platinum ngk plugs last?
 

I've got these in my car and they are nearing 5k miles now. Is it time to change them? thanks.

I've found that plugs are a once a year deal with my FD, I try to get a full day planned so that I can do the coolant, plugs and fuel filter all at the same time. I've used both Plats and non-Plats. I'd say that non-Plats are probably good for 10-12k miles. Plats are probably good for 14-18k miles, depending on how you drive. Do not change the plugs as recommended by the owners manual (I think it's 36k miles). The plugs don't last that long. Your best bet is to take them out and examine them for excessive wear. Don't forget some anti-seize when you put the plugs back or install new plugs.

When I bought both my FDs (used) I changed both the plugs immediately even though both owners said that they did them "just before I decided to sell the car". On my first FD, it didn't look like the plugs were ever changed (65k miles).