As long as your tune is on point they should last awhile, especially with Iridium plugs. Unless the car is breaking up or cutting out there is no need to change them. If it aint broke dont fix it..... your engine will let you know when its time to change them.
I would probably pull them every year just to take a look, just for something to do, but then again if your car seems to be running fine, the plugs are probably fine.