As a general rule, objects that can move or have some activity to them can be filmed longer than stationary, inanimate objects. Use lower case only and print one letter per page. Now shoot the letters, taking 3-5 seconds per letter. When you are finished, the video should be approximately 15-20 minutes long. This is optimal - long enough for the child to enjoy but not overly long so that he becomes bored.
Music can be an important component of learning as it helps focus children's attention.
Secondly, as you are filming the individual letters, make sure the sheet of paper fills the entire screen. Above all, keep things simple and fun. Don't get too involved in any one scene. The entire effort should take no more than 4 hours. If it does, you're probably being too fussy.