I'm in the auto parts business and I get asked this question a lot and I always tell customers:
"The people who sell oil say 'every 3000 miles' and the people who sell cars/trucks say 'every 7500 miles' so somewhere between those two extremes is probably the best interval for an oil change."
As for me, I usually change mine in the neighborhood of 5000 miles.
And for those of you just looking for something to flame me on, I know...I know that there are applications that call for longer than 7500 miles, but it used to be that that was the recommendation in a lot of owners manuals. So flame on!