So what is it that you see here? Back in 2010, when Apple released their first "countdown" to 10 billion downloads, I noticed that the counters on their website periodically downloaded a file to keep sure the data you see is "in sync" with the actual number of downloads. So what I do here is ask for that file every other second. It tells us how many apps are downloaded so far, when it's content was last updated and how fast people are downloading apps (the download rate). Based on that, we can easily calculate when the 50-th billion app will be downloaded. That's what I'm showing you here. The data is however delayed by Apple (up to three hours). Good luck winning the contest!
Oh, and there's another tracker, over at temporaryfix, that makes another estimate, you should check it out too:
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