Unfortunately there is no hard and fast answer to your question. Nutritional requirements for cats vary, even for cats of the same age. Factors that may affect nutritional requirements for cat food include:

• The Cat's Weight
  A 17 pound cat with a muscular build and large bone structure will require more calories and associated nutrients to maintain his weight than a nine pound cat with a more slender build. Likewise, a cat who needs to gain or lose weight will need differing quantities of food.
• The Cat's Activity Level
  Likewise, two identical 17 pound cats as described above will vary in their requirements, depending on their activity. A three year old cat who races around the house may require more than an eight year old who sleeps most of the day.
• The Cat Food Content
  Fish flavors in particular, should be fed in minimal amounts. Although most cats love the taste of fish, an exclusive fish diet has the potential for harm. The Cornell Feline Health Center says, "Diets containing large quantities of fish can cause yellow-fat disease (steatitis), a result of vitamin E deficiency." This is especially true with the lower quality fish protein often used in cat food. If your cat enjoys fish, try eliminating fish-flavored cat food; instead, give him a small treat of human-quality albacore tuna occasionally.
• Quality of the Cat Food
  As a rule, many cats fed a high quality premium cat food will eat less than they did on poor-quality "grocery store" food. Their bodies have an innate ability to judge when they have reached the proper nutritional level, and they will stop eating when they reach that level. As a result, feeding these high quality foods has the added result of making them actually more economical.
• Supplemental Dry Food
  Cats that are "free-fed" with dry food in addition to their regular meals of canned food, will eat less of the canned food.

To make it easier for you, most cat food cans and packages give you a feeding guidelines printed right on the package. Alternatively, you can check the web sites of the food manufacturers. PetSmart has a calculator for the basic foods it sells. You can input the cat's age, weight, and activity level, and it will give you the results for your specific cat.

Generally, if your cat seems healthy and active, and is not constantly begging for food, you can assume that he is getting the proper amount of food, especially if the quality of food is superior.