You shouldn't have to replace them until 100,000 k's. From what I've read, and personal experience, you should have platinum plugs in there so don't worry about it for another 30,000k's.
I got mine done at it's major service (100,000 k's) - did timing belt, plugs, oil, filters, etc, dyno tune & run. I went out and bought NGK Iridium's. Personally I didn't fell any noticable improvement with the Iridium's (despite all the hype on their website).
Just buy NGK Platinum's and don't waste your money on the Iridiums...
Cheers,
C.