The 10 to 15 year time period associated with joint implants is based on the law of averages. Using long-term studies of patients, researchers have been able to report the ranges of time that joint replacements have lasted.
We've discovered from these studies that it's not really a function of time. It all depends on the amount of use that occurs. The more physically active you are, the short the life of your implant.
There's no internal mechanism keeping track of the number of steps you've taken. No microchip to signal the end. It's just a gradual process of wear and tear. The plastic components start to get thin. The metal portions may shed tiny particles that build up inside the joint causing damage.
The first signs of problems developing may be knee pain or joint clicking. See your doctor early on before the symptoms get worse. In some cases, it's possible to revise the joint without replacing the whole thing.