I think the short answer—nobody know exactly. My 3 year old bottle of Thuricide liquid still killed caterpillars last year, so don't dump yours.
These BT formulation typically contain both dormant BT spores and the toxic crystalline protein that is produced by Bacillus thuringiensis. The toxic crystals are what stops the caterpillar from feeding and starves them to death even if the live bacteria don't penetrate the gut wall.
The powdered form is supposed to last longer, but who knows.
As long as it is keep cool and away from sunlight, any version should last for some years.
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