Are BBLs permanent procedures? If not, how long do the results normally last?

Thank you for your question. With Brazilian butt lift we transfer fat from one area of the body to the other.  These fat cells are transferred in microinjections throughout multiple planes in the buttock tissue so that they can gain new blood supply and live in their new home, providing increased volume and shape to the buttocks.  The fat that survives is there permanently. Around 70% of fat that is grafted is typically expected to survive long term. We also put a little extra to account for loss of fat post operative, which is reabsorbed gradually by the body.

In my patients, they can expect between 60-80% of the fat to survive.  The survival rate depends both on the genetics of your body and the skill of the surgeon.  Post surgery, you should follow the plan laid out to you by your surgeon, and avoid putting pressure on your backside (sitting) when possible.
My recommendation is that you find an ASPS board certified surgeon in your area who specializes in the BBL, and set up an in person consult. An in person consult with an an ASPS board certified surgeon is the only way to determine the best route for your body and your goals.
It is all about designing the perfect plan for You.
Good luck, please let us know how it goes.
-Dr. Constantino Mendieta

4-6 weeks following the BBL procedure, you can expect 30-50% of the fat to resorb. However, once the fat graft revascularises in the area of injection, it will stay there permanently. 
Since that point on, the size of the butt might vary depending on any weight gain or weight loss you experience. If you maintain a steady weight, the size would pretty much remain constant too.