The duration of the smell from a dead mouse can vary depending on various factors, such as temperature, humidity, and ventilation. In general, the odor can persist for several days to a couple of weeks. However, if the dead mouse is inaccessible and decays in a hidden area, the smell may linger for a longer period, potentially up to a month or more. Using odor-eliminating products or seeking professional assistance can help mitigate the smell.