Every time you start the car, it does a full diagnostic and if any sensor is out of its valid range the CEL will be illuminated. The CEL does not store its state between car starts.
I asked because our light came on. Since we were too busy at the time, the nearest dealership is 1.5 hours away, and there were no noticable signs of distress from the car, we let it go. A couple weeks later it turned off.
I thought maybe they programmed the CEL to turn off after so many days but that is not correct since the state of the CEL is never stored and determined upon every start of the car.
HTH someone, some day.