According to About.com, if stored properly, refrigerated feta cheese stays fresh for up to three months. It should be kept in a milk or brine bath.
High-quality cheese is taken directly from a brine solution when sold, and it should be placed in brine at home. Small quantities of the cheese are often sold in containers of salty liquids. Milk works as a substitute for brine, and it lowers the salt content and produces a milder flavor.
Feta is a traditional soft Greek cheese made from sheep's milk, goat's milk or a combination of both. The best-quality feta is aged for four to six weeks in the brine bath. As feta ages, it becomes saltier.