This is an intensely debated question without a simple answer. Some doctors suggest a rupture may happen in 10-15 years. Others remind us that for many patients a rupture may never happen. The FDA cites a rupture rate of about .05% at 3 years. Because of these figures the implant companies have recommended an MRI to scan for ruptures 3 years after implantation, and every two years following.
The rate really depends on what generation of implant you are using, as it will vary with each. Like all human made objects, implants keep getting safer and more advanced all the time. Regardless, for whatever the reason the life of an implant is not indefinite. For modern implants today, the rate of rupture is probably around 2-7% within 5 years.