Re: spark plugs
You should check your maintenance schedule in the manual for your engine. If you don't have one, you can usually find them online (I know Volvo posts them). For my engine, Volvo 5.0GL it says to replace the plugs every 3 years or 500 hours, whichever comes first. 3.0 liter models are supposed to be replaced yearly though for some reason which is why you need to check your specific application.
Wires, cap and rotor may not need it as often unless you see signs of wear. They aren't all that expensive though, so I'd probably just replace those when doing the plugs.
