How much should I feed my cat?

I got my cat when she was already a young adult, and she has always been small. She's three years old and about 9 1/2 pounds. I give her hard cat food in the morning, then in the early evening she eats half a can (small can) of the canned food. Then later at night she gets more hard food, which usually lasts her until she gets more in the morning again. Basically, she gets as much hard food as she wants, and gets 1/2 can of the soft food everyday. Is that enough? She's not skinny and not fat. I can't see her ribs and she has a small fat pooch under her stomach, but not a really fat one. She seems healthy, but I know my dad feeds his cat a whole lot more. But his cat is also overweight.