How many minutes does the shark appear on screen in Jaws?

How to get close to this number

Reason from the technique rather than from the film. A monster works best unseen, and a film that shows its creature constantly stops being frightening — so a well-made horror film shows the threat for a small fraction of its runtime. A few minutes out of two hours is what that principle produces.

The reason in the explanation is the best part, and it is an accident: the mechanical shark kept breaking down in salt water, so the director had to suggest it rather than show it. One of the most studied techniques in horror cinema was a workaround for a malfunctioning prop, and the film is better than it would have been if the machine had worked.

Screen-time counts are approximate by nature — they depend on whether you count a fin, a shadow, a point-of-view shot from below, or only the full creature. Different analyses of the same film give different figures for exactly that reason, which is why this is quoted as roughly rather than precisely.