I've had cure #1 that is at least 5 years old; and it is still good. If stored properly it should last indefinitely, though on one website that was selling Instacure it did give a shelf life of seven years. What I do when I'm in doubt I make a small batch of Basic Cure. Apply the Basic Cure to a pork chop, and cure it for about 8 hours. Rinse off the Basic cure, and fry at low heat, until the pork chop is 140°F - 145°F. If the meat inside the pork chop is pinkish, and it taste like Canadian bacon, you cure is still good.
Basic Cure (Small Batch)
1oz (28g) Pickling, or non-iodine table salt
.5oz (14g) white sugar
.15oz (4g) cure #1
Apply at a rate of 1 Tablespoon per pound.