For the spark plugs, check the owner's manual and it should have all the service/maintenance intervals in there. If you don't have a physical copy, just Google it and there should be a free PDF available. For the alternator and starter, there is no specific mileage or time that you have to replace them - it is usually just whenever they go bad. If you want, AutoZone or any parts store can test your alternator for free and check if it's still good. Unfortunately, the starter is located under the intake manifold on the Toyota/Lexus V8s so you cannot check it easily. If the car cranks and starts without problem, there is no need to replace the starter.