If you mean spark plugs, every 30,000 miles. They are not plat or double plat. The plugs are just regular plugs. Is the check engine light on? If not, then a good EFI service will work. That's if you don't have a vac leak, bad coil/coils or ?
You know rkazee you can believe what you want. Your owners manual says every 30,000 miles. scvscions would want you to spend money you don't need to spend The ignition system was designed for regular plugs not IRIDIUM PLUGS. To maintain your idle quality then have a fuel injection service done just like the Toyota dealer recommend for my mothers 2005 Scion XB, NOT IRIDIUM PLUGS. But you just do what scvscions tells you. Just because the dealer sells the vehicle doesn't mean they know how to work on them.
The manufacturer's recommended maintenance schedule is what you need to follow, and you can find that in your glove box, or on RepairPal in the Estimator: http://repairpal.com/estimator
One thing to remember is that what maintenance is needed depends on what has been done in the past, so if you have a service history, that will help. It's relatively unlikely that a regular maintenance will clear up the rough idle.