It occurs to me that if we think about how long we want people to remain "focused" on one of our images, it might tell us something about how to design that image, how to compose and frame it, perhaps even how to approach the subject. As I often do, I'll relate the photography situation to other creative endevours to see if we can learn anything.
If we relate attention to a photograph to attention to a piece of music, some of the catchiest songs on the radio are a minute long, while a symphony might be an hour. Jingles that advertizers use can be 30 seconds in length yet leaving us humming the damn thing for the rest of the day. Does this mean that a symphony is automatically better, just because it's longer? It probably takes more work to write one, but more inspiration? More genius? More talent? Not necessarily. Some of the most moving music ever written is short.
In literature, the length of the story relates similarly, from the shortness of a joke to "War and Peace". An editorial cartoon can create a powerful political message in a simple sketch and a few words, something you can get the entire meaning of within a matter of a few seconds.
I suggest that the worth of a photograph is not in how long people continue to look at it, rather it is in how much it makes them feel, or think or react.
A reaction to an image can be an accumulation of little things or it can be a big wallop in the first 10 seconds.
If you want to keep people looking, then you need to offer multiple layers of discovery. On the other hand, there's nothing wrong with a simple image with a lot of immediate impact, but which doesn't require extended viewing to fully "get".
An image which provides a puzzle will keep people looking at the image as they search for clues as to scale, subject, circumstances. Another image may make someone think , but the thinking can be done away from the image since the message of the image was quickly and clearly transmitted.
I would discourage you from trying to figure out which type of image your audience wants - leave that to the commercial photographers - instead make the kind of images which you need to make, which satisfy you. Should you pick through your images to arrange a show however, you might well decide to use some attention grabbers at both ends of the exhibit with your more thoughtful images scattered throughout.