It took Windows 219 days to hit 20,000 apps, while it took the iPhone App store 215 days to get to the same mark. The Android Market needed 419 days to reach 20,000 apps, and it took 684 days for Blackberry App World to get there.

Though this looks good for the pending release of the “Mango” software update for the Windows Phone 7 this fall, Microsoft is stressing quality vs quantity when it comes to its app store. Microsoft has been trying to bring over developers to the Windows Marketplace for Mobile by encouraging them to charge more for Windows Phone apps. They want app developers frustrated with the surplus of mediocre and free apps that dominate sales in the App Store to come over to Microsoft. Apple currently has over 350,000 apps, but it’s tough to determine just how many of these are actually worth downloading and not a complete waste of time.

The upcoming Mango updates, which will add some 500 new elements to the Windows Phone 7, should be released this fall.