Most people use sunscreen too sparingly. If you use sunscreen generously and frequently, a bottle of sunscreen shouldn’t last long. Generally, a liberal application is 1 ounce (30 milliliters) — the amount in a shot glass — to cover exposed parts of the body. You might need to apply more, depending on your body size. If you have a 4-ounce (118-milliliter) bottle, you’ll use about one-fourth of it during one application.