It depends on which car I am driving, the type of driving.
On my older cars, I change 3,000 for just city driving, and 6,000 for just highway driving. If there is a 50/50 mix, I change at 4,500 miles. If I'm driving my newer car with synthetic oil, I'll double the above figures for mileage. I don't run synthetic in my older cars. If I'm using one of my vehicle very hard, I'll triple the hard mileage use. Meaning, if I tow a trailer to the lake, which is 500 miles. Then I'll count that as 1500 miles in oil change miles.
Bottom line: If you don't want to calculate the type of driving and such, just change every 3,000 and you will be safe.