The Dark Door Test

A closed room contains one working incandescent bulb. Outside the room are three switches, labeled A, B, and C. Exactly one switch controls the bulb; the other two are disconnected. The door is sealed well enough that you cannot see any light from outside. You may flip the switches in any order and leave them on or off for as long as you like. Whenever you choose, you may open the door and enter the room to inspect the bulb. Once you enter, you cannot go back outside to test the switches again. What is the fewest number of times you must enter the room to determine with certainty which switch controls the bulb?