How long does psilocybin / psilocin stay in the body?

Once ingested, most of the psilocybin is converted into psilocin within the first hour. Most of the psilocin is then excreted within the first 8 hours...but portions remain in the system for several weeks. From Stafford's Psychedelics Encyclopedia

"High dose studies of rats suggest that psilocin taken orally is distributed throughout the body. Concentrations in tissues appear highest about half an hour after ingestion, decreasing rapidly over the next three to four hours. The adrenal glands of the test animals show the highest concentrations after the first hour, until then the kidneys have more. The small intestine, skin, bone marrow, lungs, stomach, and salivary glands also have significant concentrations-- greater, in fact, than those in the brain...

In the high dose study of rats, all but 6 percent of the psilocybin was excreted within twenty-four hours. In humans, only 80-85% of psilocybin and its metabolites are excreted within 8 hours, in the urine (about 65%), bile and feces (15-20%). Some 15-20% lingers on, stored in fatty tissues; significant quantities appear in urine up to a week later. A full 25% of the originally administered dose enters urine as psilocin."

source:
http://www.erowid.org/plants/mushrooms/mushrooms_testing.shtml