How often should motorists change the oil in their cars?

every 3-5000 miles

+5 every 3000 at the most

It depends on where you live and your driving habit.

I have changed the motor oil in my Toyota Solara every 7,000 miles for the past 234,000 miles.

Oil changing companies would like to see you and your car every 3,000 miles for a change. thats money in their pocket, but its not necessary.

My car manual states every 7,000 miles and i agree.

Be sure to also change your oil filter at the same time

I do it every three months.

If it is a new car and changing it the first time, 1500 miles. Otherwise, every 3000.

+5

I have mine changed every 2,500 - 3,000 miles.

Mine is every three years or every 30,000 miles.

There are many different motor oils on the market.
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Straight mineral oils generaly need to be changed more often, fully synthetic oils can go many times longer, synthetic/mineral blends come out in between.
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The oil container should tell you how often it should be changed.
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Note also that there will be a calendar, not just mileage, limitation. Just because you only drive to church on Sunday doesn't mean you can drive it from graduation to retirement without changing the oil!

Changing you oil actually depends on four things.
Vehicle - Age, Engine, Malfunctions, etc.
Duty - job the engine is preforming, IE Generator, car, truck, etc.
Environment - Weather, Air quality, Fuel being used, etc.
Oil - Type of oil being used; Cheaper oil brakes down faster, and therefor needs to be changed more often.
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The Standard of every 3 to 5 thousand miles is based on the car's warranty set by the manufacturer. It drop to every three thousand miles 'about' three years ago when the manufactures figured out that some people were using the lowest end (cheapest) oil and the warranties were required to cover the cost of any damage (new engine) that resulted. So now it is set for the worst case in order to cover their butts.
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To me this is rather extreme. Although the issue of changing oil too much is still being debated, so far (to the best of my knowledge) cost and time seem to be the only substantial factors.
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Here is the source that I credit the most.
The U.S. Army demands more from their vehicles than any civilian ever would, and under the harshest conditions (worse than can be experienced in the US).
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They:
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On 1 Oct 04, Tactical Wheeled vehicles in the 2300 series Federal Supply Classification (FSC) were removed from the Army Oil Analysis Program (AOAP):
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As a result, and after an analysis of AOAP oil sampling and change interval data, MAM 05-019 set the engine oil change interval to 6,000 miles or annually, which ever comes first. If vehicles already had greater intervals, the greater intervals continue to apply.
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For example, the M915-series vehicles produced by Freightliner, which already had a 10,000 miles or 12 months interval, remained unchanged.
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For transmissions, the new interval is a minimum of 12,000 miles or 24 months.
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Vehicle lube orders that already had change intervals greater than 12,000 miles kept their existing mileage intervals, but the calendar time intervals changed to 24 months.
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Tactical wheeled vehicles under manufacturer's warranty follow the change intervals prescribed by warranty provisions or guidelines. When warranties expire, vehicles will conform to the interval requirement of 6,000 miles or annually, unless the prescribed mileage/interval is greater. Greater prescribed mileage/intervals will continue.
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Desert Conditions:
However, when units are deployed or stationed in extreme desert conditions extra servicing is required. Units should refer to Section III in Chapter 1 and Appendix C of FM 90-3, Desert Operations, for desert maintenance guidance.
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Oil must be changed about twice as often under desert conditions as under US or European conditions. In desert conditions, grit that accumulates in the oil pan can be abrasive. It can also thicken oil, reducing its ability to lubricate properly. Unburned low-octane fuel can also seep down the cylinder walls and dilute the reservoir. Diluted lubricants cool less effectively, and evaporate at the higher temperatures generated during engine operation. Oil changes and lubrication of undercarriage points at more frequent intervals during desert operations will prolong engine and vehicle life.

I change mine ever 2,500 - 3,000 miles.

The manuals usually say 5,000 miles. But I'm a fanatic and do it every 2,000 miles. It must work, since I have 620,000 miles on the old thing and it still runs perfectly. I also installed a pre-lube pump that brings up the old pressure before cranking the engine. +4

As often as your owner's manual says it should be changed; maybe more often if you do a lot of towing or driving under hot dusty conditions.