For what it is worth, I messed up big time. My 21 year old daughter had a beater 2002 Suzuki Grand Vitara with a 2.5 V6 in it. Nothing but old cars for my kids, cuz they pay for half of the cost to buy them, and they are likely to dent 'em up anyway. The downside of that strategy is I have to maintain two old beater cars for my two kids.
She lives 25 miles away from me and I see her once a week. I do the best I can at maintaining her old 'Zook which actually was a pretty cool vehicle with a real 4x4 lever and low range, it was a beast in the snow. It was a good running and reasonably well maintained vehicle.
Anyway, at 145K miles, it started missing badly. She drove it that way to my house. I checked the MAF, it was OK, then I decided to check the plugs as I had never replaced them and the records that came with it (2 owner car) did not say anything about plugs. The engine in that vehicle faces forward, as God intended, and the spark plugs are cake to get to, nothing in the way at all. The iridium plugs in that thing were hammered, I am amazed it ran actually.
My heart sank though, when I pulled one of them, the tip was fragged clean off. It must have rattled around in the cylinder for awhile before going out an exhaust valve. Put new plugs in and it seemingly ran fine for another 3 months or so, but then she called crying - the car broke down, engine making horrible noise, she limped it to a gas station. I got over there, it was evident it had dropped a valve. I surmise a valve got bent/damaged when the spark plug tip broke off, and it took awhile to fail.
I called a salvage company and got it towed away, they gave me $150 for it. The whole episode made me sad, as it was so easily preventable. In the case of that poor little Suzuki anyway, 145K was the limit.
After that crummy experience, I quickly grabbed my other kid's good running '06 Mazda 6 with 120K on the clock and changed it's plugs, and they were quite worn. I think 100K is a reasonable service interval for irridium plugs, I do not want to go past that.
Kid #1 has my wife's '07 KIA now, that has 130K on it, getting the plugs changed this week, along with the timing belt, etc. Wife got a clean used one owner 35K mile '16 Ford C-Max Energi plug in hybrid, which has been a great addition, as it is the ultimate short trip vehicle, running on electricity for 20 miles or so.
All this business with the spark plugs kinda makes me glad my Canyon is a diesel, alas that has it's own set of concerns. There is no free lunch it seems
