I was told to gain a 15-20 pounds for bbl. What's the likelihood of the fat lasting for months after the surgery?

Thank you for your question. The fat last after surgery. I've seen quotes between 40 to 70% fat retention after surgery. The fat that is transferred acts like normal fat ie if you gain weight the fat grows and if you loose weight the fat shrinks.usually the fat is taken from areas where you have diet resistant fat. This fat has less of a tendency to go away when your diet. If you can gain 10 pounds I think your surgeon will find it very beneficial for your surgery. I hope you find this useful. 

I do not usually tell my patients to gain weight, because often they want to lose it after surgery and inevitably the buttocks will shrink with weight loss too.