It takes a period of about 6 weeks for the smell of a dead rodent stuck inside the walls of your house to dissipate. However, this largely depends on the size of the rodent and the methods you engage to get rid of the dead animal. Check out http://www.ehow.com/how_5485510_rid-dead-rodent-odor.html to learn more.
Okay, that is like really gross. (stomach turning). I would say it depends on the size of the animal that died (possibly a couple weeks). Why don't you try burning some incense to help overcome the smell.
You might want to get it cleaned first with bleach or something that kills the germ. The smell should go away depending on what you do to it. I am sorry if my answer solve your question. Haha but i thought it would be nice to let you know.
It takes about two weeks for the smell to dissipate. You can go to many hardware stores like Lowe's or Home Depot and purchase a pouch or a liquid you can put on cotton balls that will remove the smell immediately. We used to call it Rat Sorb.