Dead mice have a very strong putrid smell that occurs as the body decays. In some cases, the stench from a dead mouse might abate after a day, but if there is more than one mouse that died, the stench might linger longer.
The temperature of the area, the size of the mouse and the presence of humidity can all affect how long the stench remains in the area. The stench is less likely to last a long time if the area is ventilated. If the mouse can be removed, taking it out of the area is helpful. If the mouse is behind a wall, that might not be possible. In that case, opening windows and using air neutralizers might help the smell.