“How long do butt injections last?”

This is a difficult question to answer as it requires more information. "Butt injections" can mean anything from fat transfer and Brazilian butt lifting to silicone injection. Silicone injection should not be done under any circumstances, so I will skip that. Fat transfer or fat grafting, theoretically can last forever as it is your own tissue. However, as you gain weight or lose weight, you can expect to see some changes.

In general, a fat transfer can last 10+ years.

Treating the buttocks usually involves increasing volume in the upper the central region giving a nice round appearance correcting a flat central appearance. Fat grafting can be performed in many patients. The cell mass is permanent. If one gains weight, this may also get larger. Make sure that whatever is done will achieve the result in improving your shape and contour which you desire. I have combined adipose tissue reduction in the lower, lateral, and above the buttocks as well as adding volume to the upper and central buttocks to give the result requested by the patient. This is a permanent change a will change over time with normal aging.

If you're speaking of fat grafting, then the answer is permanent. If you're referring to Sculptra, that product should last you for many years.