Breast implants. There is no hard fast rule about having breast implants exchanged. They are technically not permanent implants but unless there is a problem or you need a lift/size change there is no true need to replace them. Many women will decide to change them for size, if they develop capsular contracture (scarring around the implant), or at the same time as a lift.
No. The principle most plastic surgeons use in this regard is: “if they ain't broke, don't fix them". Indications for replacement would include leakage, encapsulation, breast implant displacement/asymmetry, and/or dissatisfaction with aesthetic appearance ( size, shape, symmetry…).
No. There is no expiration date once the implants are in your breasts. As long as you don't have a problem with them, keep them.
No particular time. You only need to replace your implants if you are having a problem with them. Otherwise, keep enjoying them.
If not broke .. There is a saying that if it isn't broken, don't fix it. If you have no problems or concerns with your implants, you do not need to have them replaced. As opposed to 2004, there are now 4 different silicone implant options (saline only 2004), so there may be advantages if you want to change the implants in terms of look and feel, but again, if you have no problems, you don't need to.