I added a single jumbo shrimp and it lasted for several weeks. Put it in an old stocking or pantyhose to keep it from polluting the water when it decays, though.
Also, run some good carbon in the tank (I used Seachem's Matrix Carbon) to keep the water clear (vs yellow-ish) and it keeps the stink down, too.
You'll start to see your ammonia levels rise, then nitrites. Once nitrites start going up, the ammonia should drop to zero, and the nitrates will start to rise. Eventually, after several weeks, the nitrites will zero out and your nitrates will be the only thing left in the tank.
If the shrimp has totally decayed, add pure ammonia (that you can buy from a hardware store, but nothing with detergents/surfactents) to dose ammonia to 4ppm daily until you're ready to get the rest of the tank set up.