If they are not ruptured (MRI can assess this) and they are not bothering you then you don't need to have them replaced. If you have some concerns and want some revision of the soft tissue envelope then certainly it is reasonable to consider replacing them at the same time.
I assume you are asking when your silicone or saline implants should be replaced after reconstruction?If you have saline implants it's obvious when they rupture because you will notice an acute change in size. With silicone implants, its more difficult. The FDA recommends that you have an MRI 3 years after your initial implant placement and every 2 years thereafter to screen for silent rupture of your silicone implants. They also state that the "lifespan" of a silicone or saline implant is 10-15 years. This not not automatically mean that you have to have surgery when the lifespan is up. You should probably visit with a plastic surgeon to make sure you and your implants are being monitored and to learn about any updates/advances in implant or reconstructive techniques/technology.