There are rumors that the iPhone 6 should come out in September of 2013. However, the iPhone 5S is also rumored to be released in September. It seems unlikely that Apple will release both new iPhones at the same time. Therefore, the iPhone 5S will more than likely come out in September and the iPhone 6 will come out later in the year, or in 2014 in order to avoid long lines during the initial launch dates.
Source: iOS Doc