That depends on how much care the cat gets, the size of the cat, and the random toss of genetics.
Long-lived cats can be purebred Siamese or mixed breeds. In general, cats hit retirement age at twelve. When they get into their upper teens, they are elderly. A twenty year old cat is getting to the far end of their range.
In feral cat situations, cats are breeding at six months, getting older at three or four, and rarely make it to five. They don't get enough nutrition hunting in civilized areas, and the consequences of breeding, like fighting for males and pregnancy for females, are more than they can handle for long. They are also vulnerable to accident and disease.
So when we bring a little kitten home, we should be prepared to take care of the cat for at least ten years, and possibly twice that. We must remember we have to be able to be there for them that whole time, because at this age, getting a home is the easiest it will ever be.
And they've already used up that shot.
Read more about caring for the senior cat.