There is no set amount of time that it takes for a bird to become comfortable with you, again, every bird is different, but patience around your bird is something you will have to learn.....if the bird was afraid of the seller's hands, it doesn't sound like he had ever hand trained his birds.....
Sitting by the bird's cage, talking to it & occasionally offering it a treat, through the bars, will help it begin to trust you, but sticking your hand in the cage as though you are trying to grab the bird will just make the time longer......
While you are waiting, it might be a good idea to read about clicker training and touch training African greys......there is a lot of information about this training oon the internet and there are several video clips about it on youtube.....
Good luck.....