Decades, in most cases. And not necessarily self-neglect. Frequently it is neglect by the parents or by your environment. Currently, in many countries it is pre

Decades, in most cases. And not necessarily self-neglect. Frequently it is neglect by the parents or by your environment. Currently, in many countries it is pretty difficult to maintain healthy diet if you have below average income. The way obesity starts in most cases is that you eat the wrong things for a long time and this slowly damages your internal regulation. There isn't super firm consensus on what exactly causes this to develop, but you can bet on low protein, high sugar high carb paired with low amount of exercise and food intake spread throughout the entire day (snacking). Hyperpalatable food that is currently majority of food items in stores is essentially designed to keep people eating. But it typically takes a decade or two for most people before they visibly gaining weight. For example, I drank 2-4 liters of Coca Cola every day and for over a decade I was completely fine. Until I wasn't and started gaining weight dramatically. As far as I understand most (yes, over half) of kids in developed countries are already progressing into obesity and have damaged metabolism to higher or lesser extent. That they are not classified as sick and in need of help is only due to our medical system which essentially waits until you require medical assistance until you get classified as sick. Only some people get fat because they overeat a lot for whatever reason. Some unfortunate kids because parents let them eat whatever they want. Some unfortunate adults because maybe they are stressed and overeat to compensate. But most people really take a long time to develop the underlying condition that causes them to gain weight.