Sculptra can be used for a butt lift, but any notable size increase would require several vials of Sculptra. Regarding how long it would last, it is advertised to last about two years. One point in favor of Sculptra is the ease with which it can be done and the fact that they would be no recovery time required.
Sculptra works by stimulating your production of collagen, therefore, increasing the volume. It generally takes 3 to 6 weeks to notice the increase, and it continues to increase collagen production for several months.
Sculptra is also effective at diminishing cellulite.
Thank you for your interesting question, and best of luck to you.