It took just 10 months for the App Store to clock up one billion downloads after the store opened in 2008.
Three months later, the tally hit 1.5 billion. The next half-billion took just two more months http://www.itwire.com/your-it-news/mobility/28038-app-store-turns-2-billion , giving two billion in September 2009.
Early 2011 saw the 10 billion mark, and Apple offered a US$10,000 gift card as a prize to mark the occasion.
It seems the pace isn't slowing, as Apple has now announced a a US$10,000 gift card competition to mark the 50 billionth download.
And it's not a case of winner takes all - those who download the next 50 apps following the 50 billionth stand to win a US$500 gift card.
It is possible to enter the competition without downloading an app.