Once a rat eats rat poison it can take as little as 12 hours to as long as 3 to 5 days for the rat to die after eating the rat poison.
The more rat poison the rat eats the less time it usually takes for the rat to die and they may die within 12 hours of eating the rat poison.
If the rat only eats a small amount then it may take a few days to as long as 5 days for the rat to die.
Dying from rat poison is not instant for the rat but instead is a long slow and painful death for them.
So you might see the rat return even after they eat some rat poison and if they eat some more rat poison that you set out they should die within a few days.
When they die you'll usually smell a dead rat either in your walls or under your house or in your attic etc.