Q. What is the halacha regarding feta cheese? Is this considered a cheese that needs 6 hours before eating fleishigs?
A. Feta cheese is defined as a rich and creamy soft cheese originally from Greece, authentically made of whole sheep's milk, although many are now made with goat's milk or a mixture of the two. It has been around for centuries, modern-day, less robust versions may be made from cow's milk, skimmed milk, or partially-skimmed milk.
Feta cheese is classified as a soft cheese made of 45 to 60 percent fat whole sheep or goat's milk. The better fetas are aged (but not ripened) 4 to 6 weeks, cured in a salty whey and brine. Known as a pickled cheese, the flavor of feta becomes sharper and saltier with age. It is creamy white in color with small holes, a crumbly texture, and is normally found in square cakes. (from: about.com).
Feta cheese can be made kosher and it is indeed certified as such by major Kashrus organizations.
Ashkenazim wait six hours after eating hard cheese in the same way as they refrain from dairy after eating meat. (Remoh Y.D. 89,2).
Although feta cheese is heavily salted and stored in brine, Poiskim do not consider this to be a definition of hard cheese (Nesiv Hacholov p. 96).
Responsa Shevet Halevy (2,35) is lenient on salty cheeses even when aged over six months. (See also Rabbi Gordimer’s article on OU Kosher Org.)
Horav Shlomo Miller's Shlit"a opinion is that you do not have to wait six hours after eating common feta cheese that has not been aged six months.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit"a