How Long Is Gastric Sleeve Surgery? Weight loss after gastric bypass surgery will vary from person to person. When you decide to have gastric sleeve surgery, your bariatric surgeon will be able to estimate an amount of weight you will probably lose completely, but your individual weekly or monthly weight loss will depend on many factors, such as index. body mass index (BMI), age, gender, health, and how well you follow diet and exercise recommendations. How long is gastric sleeve surgery take after paying attention to these?