The oil change indicator light comes on when the manufacturer determines that an appropriate mileage interval has passed since the last change. Historically, the interval was every three months or 3,000 miles, but now the recommendation varies between 5,000 and 10,000 miles.
If the oil change indicator light comes on and remains on after the car is running, it is time to schedule the change within a month. If a long trip is imminent, it is wise to change the oil and oil filter before departing. Some cars are designed to provide a sort of message that the oil change is coming before the mile interval arrives.