How long after you eat something do you gain weight?

If you are in calorie balance, that is, your calories eaten equal calories burned, you won't gain any weight at all. 3,500 calories equals roughly one pound of fat. Therefore, if you create a calorie imbalance of eating 3,500 calories more than you burn each week, you will gain about a pound of fat each week. So, it happens pretty quickly. If you're talking about adding muscle, this happens at a slower rate.