Nowadays, Google app store: Android Market published a report that Google Android Apps downloads have exceed 6,000,000,000 times.
Android Market includes about 250,000 apps. There are still a big gap when compared to 500,000 apps & 15,000,000,000 downloads. However, there are data Android and Apple report, Android Apps downloads increased 3,000,000,000 times in the last 3 months and the data from apple is 5,000,000,000 times in the last 5 months. So the average increasing ratio is almost the same 1,000,000,000 times / month.
By the way, Google CEO disclosed a data on the Q2 financial report meeting. This data shows there are more than 135,000,000 Android devices and a 550,000 daily increasing.