I have a long haired cat and brush her every day. It all depends on how much your cat sheds. If a lot, try brushing every day. In the winter though I tend to brush her maybe once a week because it gets cold in my house and I feel like she needs her undercoat nice and bushy to keep warm.
How long have you had your cat? Some cats just shed more than others, but they all tend to shed most in hot weather. You may find that she doesn't need quite as much grooming in the winter.