Answer from Lawrence E. Gibson, M.D.
Sunscreens are designed to remain at original strength for up to 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. Also, discard sunscreen that is more than 3 years old, has been exposed to high temperatures or has obvious changes in color or consistency.
Keep in mind, however, that if you use sunscreen generously and frequently, a bottle of sunscreen shouldn't last from one year to the next. Generally, a liberal application is 1 ounce (30 milliliters) — the amount in a shot glass — to cover all exposed parts of the body. If you have a 4-ounce (118-milliliter) bottle, you'll use about one-fourth of it during one application.
To maximize protection, use a broad-spectrum sunscreen with an SPF of at least 15. The American Academy of Dermatology recommends using a broad-spectrum sunscreen with an SPF of 30 or more. Apply sunscreen generously 20 to 30 minutes before going outdoors and reapply about every two hours — or more often if you're swimming or sweating. Be sure to rub the sunscreen in well.