Apple tends to release the initial beta versions after a two-week interval. For example: Apple released the second and third beta versions of iOS 7 after a two-week interval, and the fourth beta version after a three-week interval. Coincidentally, Apple released the third beta version of iOS 7 on July 8th, but the first beta version of iOS 7 was released on June 10th.
According to the latest rumors, Apple may launch iPhone 6 on September 19, which could mean that iOS 8 will be released to the public on September 17th, as Apple has been releasing major iOS software updates couple of days prior to the launch of the new iPhones.