There are a threads on this subject somewhere on here. It's dependent on where you drive and how you drive and how much. City vs hwy, are you towing or wheeling, are you using dinosaur oil or synthetic. When I was driving about 1000 miles a month I would change the oil every 4 months using full synthetic; now I drive about 750 miles a week, so I change it every 5-6K oil filter included. I don't tow or wheel too often either and conditions aren't too dusty.
When you get knee-deep in a situation, floor it.