Are breast implants lifetime devices?

The short answer is no. The more useful point is what that means for a decision you make today, because an implant is the start of a long-running commitment, not a one-time purchase. This question belongs to the deciding stage.
The direct answer
The U.S. Food and Drug Administration states it plainly: breast implants are not lifetime devices. The U.S. Food and Drug Administration adds that the longer you have your implants, the more likely it will be for you to have them removed or replaced.
What follows from that
Read those two sentences together and the planning problem is clear. Time works against the implant, so plan on the chance of another decision later. A later removal or replacement is a stage of its own, with its own surgery and its own cost.
Be skeptical of any consultation that treats the first operation as the whole story.
The change that would alter your plan
Your plan changes when you move from thinking about a first procedure to thinking about the second. The American Society of Plastic Surgeons has a page on breast augmentation cost that is a place to start budgeting for what the first operation involves. Treat any later operation as another line to plan for.
The question that usually follows
Readers around Fayetteville who learn this tend to ask what the implants themselves are and what can go wrong with them. The U.S. Food and Drug Administration explains the risks and complications of breast implants, and the American Society of Plastic Surgeons describes the types of breast implants. Read both before you choose a type.