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.