Its down to use.. they do not 'degrade'
But they WEAR at the electrode from the sparking process.
As a rule of thumb.. if you get good MPG then it is telling you that
1. The motor is in good order
2. That the engine is not being heavily dtressed
If you did 5x100 mile trips a year..
That is potentially LESS wear than
100x5 mile trips.
If you are just an occasional dtiver.. then 12k is probably fine.
I think the only thing I would add to the above is that if you are frequently doing very short journeys it would benefit many systems on the car to occasionally take her for a decent drive on open roads. Getting everything up to temperature and holding it there for a while is especially beneficial for the engine oil, oxygen sensors, catalytic converter/DPF (if it's a diesel) and spark plugs. (I can think of other things too).
As to how long might the plugs last? it will depend on what type they are. (broadly speaking, Standard or "copper" - Platinum - Iridium). So far all the petrol cars in our family run "standard copper" plugs (I always buy the OE recommendation equivalent). The mileages vary between around 4'000 miles on my wife's wee Panda through 7 to 8'000 on my older boy's Punto and similar on daughter in law's Jazz up to around 12 to 14'000 on my younger boy's Astra Estate. The Panda could probably go two years and the Astra is probably about on target. Standard copper plugs are not pricey (£2.75 plus vat for the Astra's ZFR5F NGK's) and as I do all the services myself I like the dependability this gives.
The cars all get a once yearly service which I treat as a "big" service, so brakes and brake pipes, suspension, steering, exhaust condition, etc, etc, all checked. Engine oil and filter is always changed, air filters sometimes go 2 years on the lower mileage cars and the Astra sometimes gets an oil change half way through the year depending on what sort of running it's doing. I'm a bit obsessed with lubricants and many years ago I was shown a good "rule of thumb" way to check oil condition by a chap who worked on Rolls Royces. After you've been for a run take a white, lint free, rag - bit of old sheet works well - (you don't want any fibers getting into the oiling system and blocking strainers or restrictor jets) and wipe off the dip stick on it. Unless you've just changed the oil it will look dirty but look carefully. If its a bit brown/dark grey but still translucent with the cloth still easily visible through the staining? That's what you want to see. Is the staining so dark that the white cloth is badly stained but no solids/particles are in evidence? End of life, needs changing ASAP. Any sign of actual particles? Drop the oil out right now, especially if they glint and glitter because that's usually bearing metal! Diesels make their oil look very dirty very quickly so this doesn't work quite so well with them but you can still use it as a rough guide. The other "trick" he taught me was to always run the dipstick between your thumb and forefinger before reinserting it into the engine. that way you are most unlikely to get any foreign matter going back in with it.
My boy (Astra) has been extremely busy with his signwriting business so when it's service was due last summer all it got was an oil and filter change, a "wheel waggle" to look for wear and a good visual inspection for brakes, shocker leaks etc. I pleaded with him to leave it with me for longer but he was too busy! It's just failed it's MOT on emissions today! Funny that, he was just telling me a few days ago that it had a slight missfire under load! The garage are trying to sell him O2 sensors etc. I'm betting it'll be the plugs and air filter (with almost 20'000 miles on them since last changed. Luckily the generic EOBD tester on my VCDS scanner package "speaks" to the Astra so I'll be able to check their operation. However this would seem to say that around the 18'000 miles is probably as much as can be reliably expected out of the copper plugs on this car. Another problem I have had in the past is with plugs seizing or corroding in the head. In the most extreme case they can snap when trying to remove them. The old overhead cam FD Victor and CF Bedford were renowned for it but it could happen on any. The manufacturers now electroplate the plug threads to resist corrosion. Removing the plugs once a year tends to stop them seizing. I like to give them a very light smear of Copa Slip too (some advise against as it might act to interfere with the electrical earth but, in over 50 years of working on cars, I've never found it to cause a problem.
Just to finish this off I should mention that my latest acquisition ('16 plate Ibiza 1.0) has these fancy new (Platinum or Iridium) Plugs. Apparently they should last around 60'000 miles which, now I'm retired will be about 8 years! Her warranty expires this Easter and, mostly, I'll be doing the routine maintenance on her after that. One of the first jobs will be to remove the plugs and give them the Copa Slip treatment (sparingly). Oh, by the way, with regard to plug electrode gaps. Should you adjust them? If they are the long life type with the "thin" electrodes leave them alone. the electrodes are very brittle and very hard. You absolutely don't want any chance of a bit of one of these rattling around in a cylinder bore! If it's an "old" copper electrode type you can, but you might still weaken the side electrode (don't go near the center electrode). All plugs come pre adjusted out of the package nowadays so unless the packaging is damaged you shouldn't need to touch them anyway. They will have a small cardboard or plastic pipe in place to protect the threads and electrodes too. Seeing this undamaged helps to reassure that the gap is as manufactured.
