Overhead transparencies can be shown in a lit classroom, which promotes student interaction and note-taking. You can also write on transparencies to demonstrate a point. During electronic presentations using a PC, better quality LCD projectors allow for normal lighting. The surface on which you are projecting and the lighting around the screen will also affect the amount of darkness required in a room. Media Services will help you determine your needs.
Some instructors average three minutes per slide and others cover three slides per minute.
There is no hard and fast rule for timing. Students process information at different rates. Allow students time to read the slide, listen to your lecture, and take notes. If you read the slide aloud, be sensitive that you are speaking to the class and not the projector screen. Consider revisions to help you accomplish your objectives more effectively.