There are two ways to answer your question. On one hand, the longevity of buttock implants is "permanent" because the implant is a soft semi-solid silicone rubber implant and therefore cannot leak or rupture unlike silicone gel breast implants. Once the procedures is done without...
Your 250cc implants are not very large. It is unlikely that you will have noticeable sagging after removal. An image would be helpful though. Good luck!
It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation...