There have been several discussions on brine for ripening cheeses such as Feta in, most regards to using a saturated brine or 16% salt brine, the generally accepted minimum % salt to long term inhibit growth of unwanted microorganisms. There are also some Wiki Articles
on salt and brine including brine for ripening cheese in.
But most people (me included) find Feta aged in 16% or higher brine too salty for taste (unless in a cooked dish) and thus before using on salads etc often remove from brine and soak a little in milk or water first to reduce the % salt.
Most commercially made Feta is shipped either crumbled dry of with a little bit of brine, generally too small an amount to measure the salt content. So from local Eastern Mediterranean store here in Houston I asked for 3 baggies of brine with my Feta purchase, to ensure enough brine to measure the % salt. Older pictures of Feta's after removal from large shipping containers being sold by the pound here
, last pictures are of shipping containers, I bought the Bulgarian Feta. The server ladled the brine into baggies from a big bucket in a fridge, which was full of brine probably from the shipping containers of a mix of commercial Feta's. I measured it at 8.5% salt.
Also, notice the brine, looks very whey coloured, expected as this is brine the cheese has been shipped in, also notice price of this purchase at USD4.89/lb vs 15 months older linked pictures at USD4.29/lb, 11% annual inflation!
My next batch will use ~8.5% salt brine to get a less salty taste, even if it means it won't last years in fridge as less than 16%.