Most automobile manufacturers recommend changing the oil at least once a year or between 3,750 and 7,500 miles (depending on vehicle manufacturer) in passenger car and light truck gasoline engines. However, you'll discover when your read more closely that the once a year, or between 3,750 and 7,500 mile oil change (depending on vehicle manufacturer) is for vehicles that are driven under ideal circumstances. What most of us think of as "normal" driving is actually "severe service" driving.
This includes frequent short trips (less than 10 miles, especially during cold weather), stop-and-go traffic driving, driving in dusty conditions (gravel roads, etc.), driving foothill roads and driving at sustained highway speeds during hot weather. For this type of driving, which is actually "severe service" driving, the recommendation is to change the oil every 3,000 miles or six months.
For maximum protection, most oil companies say to change the oil every 3,000 miles or three to six months regardless of what type of driving you do. A new engine with little or no wear can probably get by on oil changes between 3,750 and 7,500 miles (depending on vehicle manufacturer). But as an engine accumulates miles, the need increases as blowby occurs. This dumps more unburned fuel into the crankcase which dilutes the oil. This causes the oil to break down. So if the oil isn't changed often enough, you can end up with accelerated wear and all the engine problems that come with it (loss of performance and fuel economy, and increased emissions and oil consumption).