Sunscreens are required by the Food and Drug Administration to remain at their original strengths for at least three years. This means that you can use leftover sunscreen from one year to the next.

Some sunscreens include an expiration date — a date indicating when they're no longer effective. Discard sunscreen that is past its expiration date. If you buy sunscreen that doesn't have an expiration date, write the date of purchase on the bottle. Throw out the bottle when three years have passed since the purchase date.

To keep your sunscreen in good condition, avoid exposing the container to excessive heat or direct sun. Place sunscreen containers in the shade or wrap them in a towel. Discard sunscreen that has any obvious changes in color or consistency.

Keep in mind that if you use sunscreen generously and frequently, a bottle of sunscreen won'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.