It depends on the year of the car and what type of oil you run; regular or synthetic. Newer cars run cleaner, are better sealed and thus can go longer. With regular oil you can get 3-4000 miles with no worries, synthetic oil can go 5-6000.
Older cars, say before '85 should still follow the 3000 mile mark for regular oil and I would not go much farther than 4000 with synthetic.
Other things that effect frequency of changing oil are numerous. Stop and go driving, dusty conditions, heat, cold, racing around and mileage of the motor all will change things.
After 35 years of turning wrenches I always tell folks this. "What is cheaper, $25-$35 for an oil change or buying a new motor?" When you look at it that way, changing your oil every month is pretty cheap, never mind how many miles are on it.