There are many factors that goes into determining what the attention retention rate is on a particular piece of video content. Some researchers have determined that the most important factor is the quality of the writing. If the writing and audio is good people tend to pay attention for longer periods of time even if the visuals are poor. If the writing/audio is poor but the visuals are good people tended to watch for a shorter period of time than the first scenario. To make matters even more difficult the playback medium has a lot to do with the results. Theatrical/auditorium/church audiences watching on a projection screen tended to have the greatest tolerance for sub-par content and TV and web-based distribution has the shortest retention. Radio falls in the middle.
To answer your question, the gold-standard for television broadcast story creation & production is CBS 60 minutes. In a hour show only three stories are offered. Each about twelve minutes. Mind you, that is with the best writers, reporters, videographers and audio people there are.
For scenario as you describe, if the goal is to make the congregants be aware of the new kiosk, that should take no more than :30 to :60 seconds including a compelling tease to have them try it. If the purpose is to train the congregants on the use of the kiosk, that probably cannot be done in :30 to :60 seconds and I would argue that running a “training” video in the middle of a church service (at any length) is probably not a good idea.
New York City by day & Monmouth County, NJ by night