Thought I'd just add this. My car is now on 82k miles. I'm the 2nd owner and its got a full lexus service history. On the cars 6th birthday and at 32k miles the spark plugs got changed. So those ugs have now done 50k miles. My car runs sweet. Just last week I drove to Leeds from Rochdale but in sport mode. Car stayed in 4th gear all the way. I even maxed it in 4th at 7k revs. My journeys are short in the week and I give it a run at weekends. Do I think my plugs need changing? No. Like I said my car is running sweet.
Last service, last August, I asked for a price to do the plugs. £25 a plug. £125 for all 6 I said? Yes they said. When I go again this August I'll ask again and see if it's the same price.
But my gist is plugs only need changing if there is a problem with ignition and running.