How much time will it take eating 250-500 calories a day to lose 50 lbs?

3500 calories is 1lb: 500 calories is very low, and if prescribed by a physician can be used for a short time. Pounds are lost by losing 3500 calories. So in a day if you eat 500 calories and your body burns 1500 calories through exercise and basal metabolism, you lose 1000 calories a day. Takes 3.5 days to lose 1 pound. So it depends on the calories your body burns. Varies by metabolism and exercise.