Go by the expiration date printed on the package. For most OTC medicines, opening the container doesn't start a shorter countdown — storage is what actually shortens a medicine's life.
No countdown to draw — Sunscreen goes by the date on the package, not by when you opened it.
Air and warmth are what shorten an opened item's life — the right container buys back the days the packaging gives up once the seal's broken.
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Yes — Sunscreen carries a printed expiration date and that's the date to go by. Sunscreen is a regulated OTC drug in the US. The FDA requires sunscreens to stay at their original strength for at least three years, so a bottle with no printed expiration date should be discarded three years after you bought it. One with a printed date goes by that date. Heat and direct sun degrade it faster than the date implies — if it has separated or changed texture, replace it rather than risk a burn. Not medical advice.
Below 77°F (25°C), out of direct sun — a beach bag in the sun is the fastest way to ruin it. Heat and humidity shorten a medicine's real life faster than the printed date suggests, which is why a bathroom cabinet is the worst place for it. Check the printed date and ask a pharmacist.
Throw it out if you notice: Separation into watery and oily layers; Grainy, clumpy, or runny texture; Colour or smell change; Past the printed expiration date. Not medical advice — ask a pharmacist if you're unsure.