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 — Aspirin 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 — Aspirin carries a printed expiration date and that's the date to go by. Aspirin breaks down with moisture into acetic acid — that's why old aspirin can smell like vinegar. If it does, replace it. Use by the printed date and keep the bottle tightly closed and out of the bathroom. If aspirin is part of a daily heart regimen, don't take chances with an old bottle. Not medical advice — follow the label and ask a pharmacist.
Room temperature 68–77°F (20–25°C), dry, tightly closed — humidity is aspirin's main enemy. 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: A vinegar-like smell (the classic sign aspirin has broken down); Crumbling or chalky tablets; Discoloration; Past the printed expiration date. Not medical advice — ask a pharmacist if you're unsure.