How often to change spark plugs?

i own a repair shop,and i always change them in all my customers vehicles around every 15-20 thousand miles they will last a little longer ,but to keep one running good its better to change them, so it will always get good fuel economy,, when they get a little old they break down when the get hot and don't fire real good if you change them at least every 20 thousand you,ll be fine ,good luck i hope this help,s.How often to change spark plugs?
You should do this every 25-30,000 miles in regular condition. However, you can also use spark plugs to quickly diagnose an engine and any possible problems. I like to change them out whenever I do this just for the sake of doing it. There's a guide in the back of all Hayne's Manuals that compares pictures to let you know how well (or bad) an engine is running.How often to change spark plugs?
When my car needs them changed, I change them. If I buy a used car I change them after I take posession. Check them at least every 6 months.

I do this all the time.
Spark plugs these days will last 60,000 milles with no problem. Didn't always be that way.
I change mine about every 25,000 miles. My vechicles claim the plugs are good for 100,000 miles. But that is in perect driving conditions. Changing the plugs on a regular interval is a cheap insurance for peak performance %26amp; good gas mileage. Also I don't have to worry about the plugs being seized into the heads when it does hit the time they claim I should be changing them.
Ideally every 10-15000 kilometres. Also it depends on your driving condtions, if you do a lot of stop/start, town driving, I would reccoment every 10000 and open road about 15000!
You should change your spark plugs after every 40,000 KMs of vehicle running. However these should be get cleaned after every 5000 KMs or whenever your vehicle knocks. You must check the gap of spark plug every 5000 KMs.
What kind of car? What are your driving conditions?What does the MAINTENANCE SCHEDULE in your owners manual say? Does the engine have a mis-fire?Answer these questions first,then maybe we can help you.