To find out how often to change oil in your car, you should turn to your owner's manual. Most cars these days go anywhere from 7,500 miles to 10,000 miles or more between oil changes.
Many vehicles have a calculator built into the cars computer that will calculate oil change frequency for the engine. This calculator takes things into account like temperature, crankshaft rpm's, mileage, etc. and it will make a calculation based on pre-set conditions. The it will display how many miles you have to go until your next oil change. These systems are reliable and there is no need to have your oil changed before your vehicle says it is time to do so.
Even if you don't have this feature on your car or truck just go by the oil change interval listed in your owner's manual. Changing the oil more often than the manufacturer will give you very little benefit and will cost you a lot more money over the life of the vehicle.