InsaneJournal

Level: 7th year Advanced
Subject: Potions

This potion enables the consumer to assume the physical appearance of another person, as long as they have first procured part of that individual's body to add to the brew (this may be anything — toenail clippings, dandruff or worse — but it is most usual to use hair). The idea that a witch or wizard might make evil use of parts of the body is an ancient one and exists in the folklore and superstitions of many cultures. The effect of the potion is only temporary, and depending on how well it has been brewed, may last anything from between ten minutes and twelve hours. You can change age, sex, and race by taking the Polyjuice Potion, but not species. ~ Wikia

Game Canon
+ Can be found in Moste Potente Potions, ala the Restricted Section of the Hogwarts library.
+ The larger the "bio sample" is the better.
+ Brewing length ranges between 18-24hrs depending on cauldron type
+ Polyjuice Theory is taught in 7th year, they don't brew it.
+ The ingredients needed are locked in Slughorn's office.