This is a very common question as patients want the result to last if they are going to undergo surgery.
Fat transfer has become so popular because it allows your surgeon to use your own body tissue for the augmentation. Liposuction is performed over several areas depending on the individual patient needs, and the amount of fat needed.
The results tend to last for years to come if your weight is stable. Just like any liposuction procedure, this does not prevent your body from being able to put on weight.
With the fat transfer patients typically lose about 30% of the fat that is initially transferred, the fat that is present six months post-op will remain as long as your weight is stable. If you lose weight you can lose some of the transferred fat, and if you gain weight you can increase the size of your buttock. The benefit of the fat transfer is that the fat is your own and acts just like the fat in other parts of your body. The benefit of the Brazilian Butt Lift is not only the fat transferred to your buttock but the shape transformation. This new shape will last for years as you will now have more of an hourglass figure than a square shape.
If you are at a healthy weight and maintain that you will be able to enjoy your results for year to come.
The amount of fat needed depends on the shape and needs of each individual patient, however in general at least 500cc per buttock need to be placed to see a significant difference. On average patients typically receive around 800cc per buttock side for a total of 1600cc of fat transfer. However just like a breast augmentation the amount of fat needed various from patient to patient. In your particular case I would put at least 800cc/per side for you.
When considering this procedure it is very important that you have your procedure performed by a board certified plastic surgeon that commonly performs this procedure.
I hope this helps.