I do oil change every 5000km, do plugs about every 20,000km. But I have left them longer at times and never had an issue with the coppers.
I'm sure the Iridiums are fine, and last longer. But I don't believe they last as long as they are advertised
My view is that coppers work just as well and are alot cheaper, so thats what I do. Not saying that they are the best thing out there, but they definently work for the power levels most of us are running. At the same time though, I can get to my plugs alot easier without the standard manifold and piping