Some signs that you may need to change your spark plugs are rough engine idling, hard engine starting, engine stalling, increased fuel consumption, engine misfires.
You may also notice loss of engine performance and poor acceleration as well as engine hesitation or engine surging.
If you notice any of those problems with the vehicle or you fuel consumption starts suddenly increasing and you notice you have to get gasoline more often and your gas gauge starts dropping towards empty faster than it normally would then you likely need a tuneup and new spark plugs as well.
Spark plugs typically need changed every 100,000 miles which is around every 9 or 10 years depending on how many miles you drive your vehicle per year.
Some vehicles may rack up those miles sooner than others but if it's been at least 100,000 miles since you last changed your spark plugs then it's time to change the spark plugs and do a complete tuneup or at least change the spark plugs.
If you never change your spark plugs at all then your engine may run rough, misfire, stall or be hard to start.
You might also notice that your vehicle starts to use more gasoline to go the same distance because worn spark plugs cause the engine to consume more fuel and cause the engine to lose power as well.
Other than that nothing terrible will happen if you don't ever change your spark plugs but changing your spark plugs will help improve your fuel mileage and help improve your engine performance.
Ideally spark plugs should be change every 100,000 miles as regular maintenance and for a tuneup.
You should also change out the spark plug wires, ignition coils etc and if your car as a distributor cap and rotor those should be changed every 100,000 miles as well at the same time as the spark plugs are being changed.
I've known some people to never change spark plugs and they never had much of a problem but they would usually trade in the car after 100,000 miles or so anyway.
So it was never an issue anyway.
Assuming you drive 12,000 miles per year that would mean that the spark plugs should last for 9 or 10 years before you need to change the spark plugs.