My friend had a feral cat which she did eventually tame, but it took a lot of time and patients. I'm not sure how long it took, but I'd guess a good year. The cat's name is Elvira and she would eat you up if you went near her, she was an absolute attack cat. She would always hide, but if you got too close she'd go into attack mode.
When my friend took her to the vet he (the vet) had to wear leather gloves because Elvira was attacking him. He couldn't examin her until she was sedated. She was examined and spayed. Many many months later she was starting to tame up a bit, she was still untouchable, but she was no longer hiding.
The turning point was when my friend found a tiny kitten. (I say tiny kitten, but he was old enough to eat on his own, still tiny though.) After having him examined by the vet she tried putting him with Elvira. She was very careful as she had no idea what the outcome of this would be. The outcome was amazing... Elvira bathed the Kitten (who was later named Onyx). That little kitten totally turned Elvira around. She became his "mother". It's one of the most amazing things. My friend has no idea what made her put the kitten with Elvira. She wasn't protective of him though (thank goodness). The kitten was not scared of my friend at all and would run up and play with anyone who went into the room, eventually Elvira started to follow the kittens lead.
This was a few years ago, now Elvira and Onyx are just as inseparable as ever. They are both very tame and absolutely wonderful cats. Elvira still hates the vet though, haha.