Aubrey de Grey (and others, like Chris Lawson) have suggested that radical life extension using technologies that could be attained in a matter of decades would lead to life spans of 5,000 years or so. Where does this figure come from?
Once our medical technology has passed the stage of what Aubrey de Grey calls acturial escape velocity - the point at which the effects of aging are being defeated faster than we age - our life spans will be limited only by accident and violence. The odds of death under these circumstances are very well researched for our current Western societies:
The odds of dying from an injury in 2001 were 1 in 1,781.
If you take the statistics for a healthy person in the prime of life and keep calculating the odds of death based on present day statistics, you wind up with a projected life span of thousands of years. As Chris Lawson says, it is perhaps more appropriate to consider ageless individuals to have half-lives in the same fashion as radioactive elements:
[An ageless man] who has lived 500 years has a 50% chance of living another 500 years. And should [he] survive that to reach 1000 years of age, he still has a 50% chance of living another 500 years. This is always true: no matter what [his age], he has a 50% chance of surviving another half-life.