Every year, or as an when the service requires
Replace them every 100,000 miles.
Your spark plug manufacturer would recommend your replacement schedule.
You should replace the spark plugs when your car won't start first time but does eventually.
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+.
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).
The owners manual says every 75,000 miles
6000 miles
How do you replace the spark plugs on a Peugeot 307?
most spark plugs should be checked and gapped annually. if your spark plugs are used up you should replace them
How to replace spark plugs in a 1989 jeep cherokee?
Generally, to replace spark plugs on a Subaru vehicle, the spark plug boot should be removed. Then, the old spark plugs should be removed using a socket wrench, and replaced with new ones.
To replace the spark plugs on your Celebrity requires a 5/8" spark plug socket, short extension and either a ratchet or breaker bar. First remove the spark plug wire then use the socket to remove the spark plug. The spark plugs should have a .045" gap. Replace the spark plug and reattach the wire. Continue until you have replace all of the spark plugs.