The world first hit the billion person mark in 1800. It took another 130 years to add another billion people. From then it's been gangbusters thanks to advances in health care and food production. Since 1968, the world's population has doubled from 3.5 billion, and at its current clip the human population adds a billion people every twelve to thirteen years.
As the New Yorker noted in a recent article, academics and scientists -- beginning with Thomas Malthus in the early 1800s -- have suggested that disease and famine will at some point control the human population. The question is: When?