Before making any major iOS release public, Apple has always released the beta version for developers. However, there was only one version of iOS which never went into beta because Apple didn’t had any iOS SDK or App Store in 2007. Therefore, in the comparison chart below, WillHains has compared the release of each iOS beta, starting from OS 2.0 and no. of days it had lasted.
If you observe the chart keenly, you would realize that iOS 5.0 had been longest in a beta release. Apple seeded iOS 5 beta on June 6 2011 to developers which lasted for about 128 days since its release and had 8 beta version. On the other hand, iOS beta that lasted for only 60 days is iOS 2.1 with only 4 beta versions. It seems that after the release of iOS 5.0, these bars for the seed of iOS beta would break even the previous records. 
Nevertheless, we could be wrong here.  Perhaps, this could turn out to be shortest releases of all iOS Beta in the history of Apple. Hence, nothing can said at the moment. If you are a developer, you could go ahead and install iOS 6 Beta 2 from the developer portal. All the data for the comparison chart was originally taken from Wikipedia prior to compilation. Also, non-developers are advised not to pirate the developer firmware and install it on their devices.
Let’s know your thoughts in comment section.
iPhone / iPod touch downloads