Despite the news that the iPhone 5C might not be so "low-end" after all, WSJ is still reporting that Apple has asked Foxconn to ship "both a new high-end and low-end iPhone in early September". Apple has tended to release devices to market within a couple weeks of announcing a new product, so that could mean that the new iPhones will be available for purchase by the last week in September.
An earlier report suggested that Apple might release the iPhone 5C even earlier in China, if the deal with China Mobile goes through. Otherwise, the projected iPhone lineup is the iPhone 5S at $199 on-contract (~$650 off contract), the iPhone 5C at $99 on-contract ($400-500 off-contract), and the iPhone 4S for free on-contract ($200-300 off-contract). The iPhone 4 would be dropped altogether.