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.
I remember creating the full list of ingredients for the Polyjuice Potion. Each one was carefully selected. Lacewing flies (the first part of the name suggested an intertwining or binding together of two identities); leeches (to suck the essence out of one and into the other); horn of a Bicorn (the idea of duality); knotgrass (another hint of being tied to another person); fluxweed (the mutability of the body as it changed into another) and Boomslang skin (a shedded outer body and a new inner).
The fact that Hermione is able to make a competent Polyjuice Potion at the age of twelve is testimony to her outstanding magical ability, because it is a potion that many adult witches and wizards fear to attempt.