Implants do not have to be replaced after any specific time. But you have to assume that somewhere down the line you will likely need an implant exchange for any one of many reasons.
Do you always need to replace #breastimplants?
Generally, I advise my patients that although implants are not "lifetime" devices, if there isn't a problem and they look/feel good then let them be. The older generation of implants tend to weaken with time and become at risk for rupture (meaning current implants over 10 years old or so) and should probably be exchanged for a current generation implant which have sturdier shells that don't tend to rupture in most cases. Of course, it is important to stay up-to-date with breast self-examinations and screening mammograms. Good luck and please remember to always consult with board-certified plastic surgeons for best recommendations!
Replacing Breast implants?
If there is no indication for replacement - implants can remain. There is no definite requirement to replace intact implants that have not caused or produced a problem. The most common reason that I see for replacement is a desire to go larger, and then rapture in order of frequency.
Newer style implants do not need to be replaced a fixed number of years from the date of implantation. As long as they are intact, not encapsulated, and you are happy with their look, you can keep them in place. For gel implants, an MRI every 4-5 years should be done to ensure the shell is intact. Best wishes, Dr. T.