Every 30,000 miles or 24 months, depending on how much you drive the car and on what conditions
Every year, or as an when the service requires
Replace them every 100,000 miles.
The owners manual says every 75,000 miles
Depends on the type of spark plugs. Factory plugs around 70 - 100k depending on the condition of the plugs. I would replace the spark plugs out with some AC Delco iridium plugs they will last you 100k+.
Your spark plug manufacturer would recommend your replacement schedule.
If you do not have any problems with plugs, change them when you replace the timing belt (do not forget to replace the spark plug wires too).
You should replace the spark plugs when your car won't start first time but does eventually.
Replace the valve cover gasket.
6000 miles
it's a 4 cylinder car, right? so it should have 4 spark plugs.
Disconnect the car battery cable, remove the spark plug cables, remove the spark plug boots, and remove the old spark plugs. Next, put in the new spark plugs, replace the boots and cables and reconnect the battery cables.
You should replace your spark plugs at about 60,000 miles if you drive your car rough or as they call it "lead foot". If not you should be able to get about 90,000 out of them. Just remember that the sooner you change them the sooner you can get power and fuel economy back.