How long does it take for a bird to trust you?

I just got a three month old green cheek conure on Saturday. He came from a breeder who was highly recommended to me by some bird friends. He is my first bird. He is very sweet. He will let me pet him all over. He flies to me and preens my hair. We just have one issue. He bites. I don't think it's regular GCC nipping. He bites my hands very hard when I try to get him to step up. He doesn't just bite once, but the same spot quickly and very hard over, very hard. I have welts all over my hands. He bit through the skin on my knuckle. I've been told that he's just testing me and it will take some time for him to trust me. How long does it usually take? I don't know how much more my hands can take.