After a rat ingests the poison it can take 2-3 days for it to die. Immediately a rat ingests poison, it stops feeding which reduces the risk of secondary poisoning...
Is it a 1st or 2nd generation rat poison? When buying rat poison, you will likely come across 2 categories. The first generation kills rats slowly and they will need to feed on the poison multiple times. The second-generation poisons act fast and the rodent only needs to feed once for it to take effect...