Straight from Wyeast Labs website:
Best if Used by: This package is best when used within 6 months of the manufacturer’s date when stored between 34-40°F (1-4°C). Older yeast or yeast that has been exposed to higher or lower temperatures may take longer to become active or swell.
But no idea how long they can go without being refridgerated.
GREEN TRUCK BREWING CO.
"Brewed Fresh from the North End of Puget Sound"