Butt implants, unlike fat injection do not disappear. Fat injection in large quantity, does not survive and completly disappear or almost completely disappear after 1 year. The silicone implants are non bio-degradeable and since they are soft solid , they do ot leak or break. Theoretically the can degrade over time, however in over 10 years of performing butt implants surgery , I did not witness it. So, the only reason to replace them will be if they break or deteriorate. Most importantly, consult with a board certified plastic surgeon that you trust, who performs lots of butt implants surgery and you like his or hers results.
Best of luck,