Luminol can work even in a situation that is more than 100 years past the time blood was present. For example, it has been used in the Lizzie Borden house not too many years ago to examine the basement area for blood. It can work in a situation where the room has been repainted or refloored. The main issue would be if someone cleaned the crimescene with bleach. In that case, the whole room would glow as though it was covered with blood - however, it is easy for an investigator to distinguish between blood and bleach due to the characteristics of the two.