Kate, generally fat is stored as fat so it comes into the body in its storage form. it is taken to the liver and then around the body to be deposited, I would say that it is done within minutes, certainly not more than a couple of hours. If you were to eat a fat free diet (which is nearly impossible) you can convert carbohydrates and protein to fat through a process called de novo lipogenisis. I don't know how long this would take, but it isn't very efficient and although it is physiologically possible, your body will normally just burn the carbohydrates and store more fat since we always tend to have a little of that in the diet. I also think that it's important to point out that a fat free diet isn't necessarily a good diet and if you are eating a healthy diet that doesn't contain more calories than you will be expending, you will not store a significant amount of fat.
Veronica asked about weight gain and aging. As people age their basal metabolic rate decreases so you burn fewer calories to maintain life. To avoid weight gain you have to either eat less or exercise more (actually both is the best option). Exercise is important to maintain muscle mass (which decreases with age) and thereby minimize the decreases in metabolic rate. Eat right and you'll be able to maintain a healthy body weight. I should point out that this is easier said than done. We develop eating habits that are hard to change, yet our bodies change requiring that we make some changes.
Vertigo asked about recent milk studies that related milk consumption with weight loss. I've seen them, they do exist. First off, all of the studies included a calorically deficient diet that included milk. With or without milk, if there is a caloric deficiency you will lose weight. I know, they showed that those drinking milk lost more than those who didn't. It's also important to recognize that drinking milk is generally recognized as a healthy behavior because it is associated with drinking less soda. People who follow "healthy" diets also consume more fruits and vegetables and fewer meats and sweets. There's more to this than just milk. It is also important to point out that every study I've seen has been funded by the milk industry in some way which leaves them suspect. I'm not convinced, but I could be wrong. Here's another cartoon from that great siteTaylor and Emily have adopted the diet... Good work. It's hard when your spouse isn't completely on board. The vast majority of Americans could see huge health benefits from changing their diets. They are unwilling for whatever reason. Instead people take all sorts of pills to make them feel better and have more energy, etc. A good diet and regular activity will do that for you at far less cost and it works even better than drugs.
If I missed any questions, let me know.