|MadSci Network: Environment|
I'm afraid your question isn't really answerable in any meaningful way. Us humans are yet to come up with a reliable way to predict the future, and any guess that one might make will rest entirely on big pile of assumptions.
The point at which the human race can no longer be sustained by the earth is called the Omega point. Luckily, there are number of people who have made guesses on when the human population on earth might end. A lot of those people have posted their estimates on the internet, and you can find several by looking at this site: Omega, or by checking out the results of this search from google.com.
If you have a look at those sites, you'll see that there is a very large range in estimates for the lifespan of humanity. You'll also find that lots of the people who made the estimates are less than impartial!- I'd suggest you keep that in mind as you surf.
Hope that helps!
Rob Campbell, MAD Scientist
Try the links in the MadSci Library for more information on Environment .