How often do you guys change your plugs?

I drive my car about 8k a year. 70% of that is highway and 30% intown, with around 6 trips to the track a year. The car is stored once a year for a good 5 months as well. It has a 383, and will soon have the D1SC on it. How often should I change the plugs? How often do you guys change your plugs with blown applications?