According to the Mayo Clinic, most sunscreens are designed to last for up to 3 years. Some sunscreen products also include an expiration date and should not be used past that date.
It is important to keep in mind that when sunscreen is used daily and liberally, it is unlikely to last into the next year. The American Academy of Dermatology recommends using a sunscreen with an SPF of 30 or higher to truly protect the skin from the sun's harmful rays. It is also important to apply sunscreen a few times per day when spending an extended amount of time in the sun.