Answer Wiki. It depends on the type of rat poison involved. The three most common types affect the blood, the brain, and the kidneys respectively. The one that affects the blood causes an animal to bleed out after ingestion, and it takes 3-5 days before it starts to take effect in an individual who ingests it. The one that affects the brain causes neurological signs like seizures and weakness. It takes from 12 hours up to 7 days before signs are seen.