Two. One to secure the ladder firmly as to prevent harm to the other, who twists the bulb into the socket in a clockwise fashion.
Q: How long does it take M*W to screw in a light bulb?
A: The task is never completed; the partitioning of her day does not allot enough time to complete the job.
8 hours : sleeping
3 hours : eating, washing, restroom
6 hours : bashing Christians on sciforums
2 hours : ranting about how Jesus got married, had kids, and went to France
2 hours : ranting about how Jesus never even existed
3 hours : trying to dig herself out of the resulting hole