Most commercial sunscreens are made to maintain full strength for three years. However, some are not made to last that long and have a printed expiration date on the packaging. After the expiration date, sunscreen begins to lose its effectiveness.
When consumers buy sunscreen without an expiration date, they should use a marker to write the purchase date on the packaging so that they know when it reaches three years. Users should apply sunscreen 20 to 30 minutes before going outside in the sun. Sunscreen that is applied lasts for anywhere from one to two hours, depending on the SPF and the wearer's activity.