It can depend on how many poison pellets the mouse eats and what type of poison is in the pellets. If the rodent eats just one or two bait units, then it usually takes three to five days until they die. However, if a mouse or rat has eaten more than 10-14 bait units, they will most likely die within 24 hours of ingesting the best rat poison.
Keep in mind that it varies by rodenticide product, as some types kill rats quicker than others. So make sure you know what type of bait you are using before applying it to ensure that all rodents are killed fast enough without suffering too long.