Okay...this is the 'sixty four thousand dollar' question When should you change your Spark Plugs and Leads? Quick answer: 'Before it gets too late and costs you a fortune in Garage fees!' But that is not what you want to hear so this is a recommendation only.
There was a time when driving 10.000 miles per year was the accepted fact, but these days the average mileage per year has increased to at least half that again if not double. We have this huge tendency to 'overuse' our cars but this is what society says is the 'norm'. How often do you (and I hope you do or are you feeling guilty already...) check the oil in your car?
Now I know you are feeling guilty...well if you're not I am! You should check the oil at least once a week or maybe bi-weekly and make that a habitual thing to do, just as you would like to keep your car pristine clean! To get back on tract though the changing of your Spark Plugs and Leads some recommend that you at least check them every three months and change them every six months but I suppose that really all depends on how much your car is used. If you have had your car for a while and use it just 'locally' and all of a sudden the urge is there to go and visit Momma in the next State to yours, then before you start this journey...check the oil and water...and hey what about checking the Spark Plugs and Leads then what a good idea...why didn't I think of that one...'duh!!!' This precaution will certainly prove to be wise and in the end can save you a lot of money if you really need to do it and you ignore it.
Once you have got the Oil; Water and the few tools needed (for checking the Plugs and Leads) all neatly laid out in front of the car you could do all that in let's say about half an hour...unless that is you have that favourite cup of yours filled to the brim with coffee and your neighbour ambles across to ask you what you are doing. You can become the 'bright spark' (then excuse the pun) then and tell him (with your chest all 'puffed' out) that you are doing a service on your car and it is saving you a HUGE amount of money. Who knows you could soon see your whole neighbourhood with their heads under the hood of their 'great' 'fantastic' (run of the mill) car!