Paint doesn't really have a hard expiry date — it has a storage problem. A well-sealed tin of emulsion or water-based eggshell from Farrow & Ball, Little Greene, Dulux or COAT will comfortably keep 2 to 5 years once opened. Oil-based paints and solvent-borne finishes often last longer still. What kills paint is exposure to air, contamination and temperature swings, not the calendar.
The enemies are simple. Air dries out the surface and forms a skin. Bacteria and mould get in if water-based paint sits half-open in a warm shed. And freezing wrecks emulsion permanently — once water-based paint has frozen and thawed it goes grainy, separates and never recovers. So never store paint in an unheated garage over winter.
To store it properly: clean the rim before sealing so the lid seats flush, tap the lid down with a rubber mallet (not a hammer — you'll deform it), then store the tin upside down. That puts the seal at the bottom and any skin forms on the base, leaving good paint on top. Keep it somewhere cool, dry and frost-free.
Before you reuse a tin, the test is straightforward. Stir it well. If it stirs back to a smooth, even consistency it's grand. If you get lumps, a sour or rotten smell, a rubbery skin you can't break down, or it's separated into a watery layer that won't recombine — bin it. No amount of stirring brings back paint that's gone off, and using it means a patchy, bitty finish.
Worth noting: premium brands tend to keep better than cheap trade emulsions because the binders and biocide packages are more robust. A part-tin of Setting Plaster or Pointing stored decently will still touch in cleanly two years on.
Practical tip — decant the dribble at the bottom of a big tin into a clean jam jar for future touch-ups. Less air space, better seal, and you'll always have your exact colour to hand.