What you mean is what is the biggest deficit you can be in and not lose muscle/lean body mass (LBM). We say this because there is no starvation mode, only the myth and a bad excuse for not losing weight when supposedly dieting. First of all, any time you lose weight you lose both muscle and fat (1/4 muscle, 3/4 fat) unless you supplement and weight train. BUT even with weight training you can't lose any more than .7% of weight/week without losing muscle. For example, a person weighing 200 lbs can lose 1.4LB/wk (~750c/d deficit) - BUT they must supplement at least a daily multivitamin and mineral (MVM).
The greater the deficit the more you need to supplement, meaning ideally to preserve or increase LBM you need you need to consume a diet consisting of ~500-1000 fewer calories than you burn, a MVM, and a pre/post workout formula. This is what all dieters should do because the goal is to increase LBM during weight loss to help burn more calories, not less. This can help offset the fitness/weight loss paradox: "The more fit and lighter we get, the fewer calories we burn when all things are equal and thus end up at plateaus, which forces us to make an unwanted adjustment.” If you want to lose weight very quickly, but safely, use the dotFIT program. Create your goal and you will receive the ideal plan to get you there.