Apple has announced that its Mac App Store has topped the 100 million downloads mark less than a year after launching. That pales next to the company’s iOS App Store though, which is now past the 18 billion mark, and is generating more than one billion downloads a month on iPhone, iPod touch and iPad devices. Apple also says that there are now more than 500,000 apps available in the iOS App Store. But how hard is it to reach the top of the charts in that store if you’re releasing a free app, as many music companies are? Analytics firm Distimo has published some research, claiming that in the US, 80,000 average daily downloads are required to become a top 10 free iOS app, although that falls to 18,000 in the UK, 12,000 in Germany and 10,000 in France.