How many grams of fat should you have a day?
This is a very open-ended question, similar to "how long is a piece of string?" The answer depends on your goals (fat loss, maintenance, or fat gain for a minority of people). It also then depends on your energy output versus your energy input, and the speed of your metabolism.
If your goal is fat loss, then many leading fat loss nutritionists recommend that you reduce your fat intake to 30 grams per day. This is quite hard, but effective. However, you can screw this up quite well by increasing your carbohydrate intake (ie sugars & starches & grains), only because energy in vs energy out is what dictates your fatness. The fat free versions of most foods have just put more sugar in instead to keep you interested. Some endurance athletes comfortably take in 100 grams or more of dietary fat, because there energy output is high enough to burn it all up. For body fat maintenance and general good health, aim for somewhere between the two. You need fats to survive. Nerves and brains are made of fats. Never eliminate them completely.
If you have minimal energy output (you drive everywhere, you do no exercise and sit at a desk all day) your overall energy intake should be low too, which includes fats, proteins and carbohydrates. If one is high, then the other needs to be high too. When there is a surplus or a deficit, you will either gain or lose bodyfat.