Right now we are in the beta period of iOS 6, specifically with the beta 2, but how long until beta 3? When will the final version come out?
In the image you can see how long has each of the betas lasted of each iOS. The beta that lasted the least was that of iOS 2, at that time it was called iPhone OS; lasted exactly 50 days, instead iOS 5 was the one with the longest beta, 128 days and eight betas plus the GM version. Everyone has had at least 3 betas except iOS 2, which had some betas that lasted a long time.
Most likely iOS 6 betas be like iOS 5, seven or eight betas, since the final version will be released with the iPhone 5 (or whatever you want to call it) October about. We still have a few days of beta 2 (you can download it here) and many more updates until the final release of iOS 6.
More information - Tutorial: how to install iOS 6
Source - iClarified