I always recommend using shorter lectures for online classes, and that seems to be the trend. The Chronicle of Higher Education says that many online instructors who record their lectures have found that 20 minute segments work well. It's a different situation in the face to face classroom, but even so, I know that I am not usually spellbound by long presentations unless the speaker is well-organized and has prepared their presentation well. With podcasts, the ones I listen to most frequently are around 30 minutes long, or if they are longer, I listen to them in two sessions.
Tools and tips related to online and distance learning and using technology in instruction