+1 for Dudesky.
I change mine every 15K or two years.
And celsdogg, they also say to change the engine oil every 7500 miles, but I think we all know not to do that. I actually change my engine oil every 2500 miles (4-5 months), since my car sits a lot.
Also, $15 per plug is really high. Just go to AutoZone or whatever you have in your area. They have Bosch +4 Iridiums for like $8 a plug. I just run the original one electrode plug, never had any problems with that.