It has been customary for Apple to hold their World Wide Developers Conference (WWDC) around mid-June, but Apple broke that tradition in 2011 in favor of the iPhone 4S which was released in October. Current rumors suggest that Apple will return to their summer schedule for the release of the much anticipated Apple iPhone 5 and iOS 6.0.Many customers hoped that the successor to the iPhone 4 will be a redesigned iPhone 5, but Apple chose to stick to the same outer design of the iPhone 4 when they released the iPhone 4S. It is now expected that Apple will release the iPhone 5 with a larger (at least 4-inch) display in a redesigned body and powered by a quad-core A6 processor. The device would make it's debut at the WWDC 2012.
If these rumors are to be believed it would mean that the iPhone 5 would be released just 10 months after it's predecessor. Whether Apple chooses to release their next iPhone in June or October, we can be assured that it will continue to be as successful as it's past siblings.
source - MacRumors