Like Ray, I change my plugs atleast 4 times a season. If I get one random misfire code or the car breaks up in any way it's time to change them. Since I no longer put alot of miles on the car waiting every 3k miles is too long.
AJ here is a good comparison for you with a big cam. This was my car pre fast 90/90 and new cam. My mods are very similar to yours just the cam is different. The TQ/HP curves are very similar below 4k but look at the differance above that. There is alot more left in your motor.
http://i165.photobucket.com/albums/u57/playwithlinn/LinnBeforeandAfterMotor.jpg