How often to change N20 plugs?

I've had autolite 103 plugs in my car for almost 1 year now (car is parked 7 out of 5 months for winter thought). The plugs have about 7,000 kms on them and maybe 5 full bottles of juice have ran past them.

When should I be changing plugs?? do you think I should switch to NGK TR6s??

I put a set of plugs in every time i go to the track, spark is very importaint for safty and consistancy. but it depends how often you race, the tr-6 last about 10-15k miles at most, then they should be changed no matter what. id say for 15 bucks and an hour of time, change them every or every other oil change. just my opinion.