What year corolla do you have? The new ones need to be changed every 5K miles. The dash gives a nice little reminder light that needs to be reset when the oil is changed.
I have a 2005 with the maintenance reminder and it flashes at close to 7.5k miles and then when you reach 7.5k mile intervals it stays on continuously. According to my Passport to Performance (the Corolla maintenance guide), the oil only needs changed every 6 months or 7.5k miles. That is the schedule I follow, and the oil that comes out after 6 months really isn't that dirty.
I have to change based on time because my car sits parked a lot and then when it gets driven it gets driven a lot. Actually, I guess I follow something closer to 5k oil changes the more I think about it. I had my oil changed for the second time at 5.3K, then I reset the light when it came on at 7.5k. At 11k and 16k I had the oil changed also, and reset the light when it came on at 15k. I'll get the oil changed at 20k and reset the light again at 22.5K.
BTW, the light can be reset quite simply with no tools. The procedure is outlined in the owner's manual, as well as on this website in several posts.
To reset the maintenance light, you follow the procedure in your owner's manual. I'll outline it here for convenience
1) Get into the car, make sure all doors are closed
2) Turn the key to "ON" but don't start the car
3) Press the odometer reset button until the odometer (not the trip meters) is displayed.
4) Turn the key to "OFF", but do not remove it.
5) Make sure your dimmer switch is set to the door position (that is anywhere other than on or off, but don't worry about this step because nobody leaves their dimmer in the on or off position)
6) Press and hold the odometer button.
7) Turn the key to "ON" while still holding the odometer button.
8) Six zeros will appear, keep holding the odometer button until step 9 is completed
9) The zeros will disappear one by one until they are all gone.
At that time, the total mileage will be displayed and your "Maint Required" light will go off.