This depends on each patient and a variety of factors. It depends on your weight, your activity level, health and strength. The more stress you put on the knee, the less time the knee will last. That being said, there is an optimal amount of activity for a knee to work best and smoothly. In general, knee replacements can last about 15 years. You may only have 2 replacements on each knee in a lifetime. This is whey surgeons will hold off giving you a new knee as long as possible.