I found this at http://answers.canadiantire.ca/answers/9045/product/0591935P/wilson-wilson-wilsarin-rat-and-mouse-bait-pellets-questions-answers/questions.htm

Wilson Wilsarin Rat & Mouse Killer does not have an expiry, as long as it is stored correctly in a cool dry location, away from moisture and excessive heat. It may seem difficult to kill rodents with baits because their feeding habits reflect there scavenger eating habits. They will eat a small amount of something and wait to see if they will get sick, if they don’t get sick then they continue to feed. This means that baits must be slow working. Also due to new Federal regulations that will go into place as of as of December 31, 2013, for domestic class anti-coagulant type rodenticides, we presently have two different active ingredients under the Wilson Wilsarin Rat & Mouse Pellets label.

The original active ingredient is Bromadiolone (059-3616-0), which is a single-feed bait. It is an anticoagulants that leads to the disruption of the vitamin K1 cycle, resulting in the inability to produce the essential blood-clotting factors, where death occurs after 1 - 2 weeks post ingestion of the bait. It work on a body weight to bait weight ratio where the rodent must ingest one meal equaling 1/4 to 1/3 of their own body weight. Once a lethal dose has been ingested, it takes 4-5 days for the anticoagulant disruption to begin and they should be dead in 12-14 days. If you are not putting out enough bait or if the bait is being taken and stored the results will be less than ideal. If the rodent droppings have changed to the same colour as the bait then it is a clear indication that the process has begun.