From: Charles Greathouse <charles.greathouse@case.edu>
The doomsday argument suggests considering oneself as a random sample from the total number of humans to be born. Estimating the number of humans born so far at 100 billion and the number of annual births at ~0.13 billion per year (assuming stabilization around 10 billion, if current trends continue) a 95% confidence interval suggests that humans will be around at least 20 more years but not more than 30,000 years. Adjust assumptions to suit.
--That argument yields very different conclusions if we assume that future "humans" (which by the way might not be considered human) will have much longer lifespans. For example I think cybernetic "life" organisms will become important and their lifespans could be comparable to the lifespans of stars. In that case, assuming you are near-median chronologically does NOT at all imply that intelligent life will end in the next 100K years.