How long should it take for my cats to get used to each other?

We got a 3 year old male neutered cat last summer. He's a doll and very sweet. He is a Maine Coon or at least a mixed Main Coon. We got another one from someone yesterday, a 2 yr old girl who is spayed. They almost look like twins! Anyway, our boy is sweet as can be and is very interested in her. But she keeps hissing and won't go anywhere near him. How long should we expect her to keep this up? Do some cats never get used to living with other ones? They haven't fought yet but she doesn't like being around him and gets jealous when we pet him.