Usually it does take a few days for them to die and sometimes they do die in the walls. They are so small, there is rarely much of any smell.
When you use mice poison: wont they walk off and die anywhere? what if they die in the walls and start to smell or something? or somewhere els you cant reach them?
The idea of the poison is "supposed to be" that the mice (or rats) get so thirsty from the poison that they go search for water. This was something they added to the originally made poison compound (and you are supposed to remove water sources such a pet bowls of water) so they would go outside searching for water and die outside. They did this change because they actually were dying in attics and walls and also killing any household pets that ate them.
But my friend used the newer poison years ago and a few did die in the walls and the smell was so horrid no one would go over there. He had to take the sheetrock out and get the few dead ones and then re-sheetrock. I have heard many of these stories! Also squirrels will eat the poison and die in the attic if they've made a hole to go nest in there.
Really the best way is to buy several traps and be diligent and just trap them one by one unless you have several families of mice. The younger mice are easy to trap because they haven't learned to get scared yet, but you have to get ALL FORMS of food cleaned up from carpets or floors to make sure their only choice is the tiny bits of peanut butter on the trap.
I had a family of 5 mice that were eating dried cooking herbs of all things from my kitchen counter. Since that was their only choice of food, they had a path they were used to following and I caught the whole family in one day. When one mouse didn't come back, I reset the trap and another came for the herbs and got trapped too. (I wouldn't not look at or get rid of the mouse in the trap-I made someone else do that part -haha)
If someone has that many mice, they have a food source. Most times it's dried dog food or crumbs left from eating around the house instead of just in kitchen or table.
I am sorry this was so long.
When I was a kid and we used poison on rats one got in the walls and died. It stunk to high heaven! But they were river rats and there were about the size of a small cat. Mice are much smaller and wouldn't smell for long or that much.