On the ol' Fairmont and Zephyr, I try to go about 3k, I'll change at 2750 if I know I'm going on a long trip. I'll let it slip to 4000 if I'm busy.
On newer cars (less than 150K and running well) I do between 4500 and 5000.
My ex, on the other hand, doesn't change her oil ever. She put 15k on a new Subaru (her first and only new car...years before we met) and it was alive still when her mechanic told her to sell it quick! ("You're supposed to change the oil?" was her question).
When I was at sea last time (right before I got laid off) she borrowed the Fairmont and put about 6000 miles on it; I told her it was due for an oil change but she forgot. I came back and in the course of getting laid off and dealing with several crises I put another 5000 on (in 9 days!)--14,000 miles on that load of oil; it used 4 quarts in that time! When I finally figured out that the oil wasn't done right before I got home like I asked, I was understandably upset. :/
Not bad for a 350,000 mile engine. It still has 155 lbs all around (+-10). Now if I could only get the poor thing over 17 mpg (higway...it'll brush 19 if I drive 80 to 85 for a long distance...that's the best it's ever done! Take that environazis!).
I record all service, fuel, & etc. in a logbook, including MPG and unusual trips. 8)
Ben