No leaked diagrams, no leaked prototypes, no leaked shells. But the rumors are already afloat.
Update [29-August-2014]: Apple Sends Invites for September 9 Event: Gear Up for iPhone 6, Possibly iWatch too. Read more.
Update [06-August-2014]: Apple Holding a Media Event on Sept 9, Could Release iPhone 6.
Update [01-August-2014]: Apple Store Employee Reveals Info About iPhone 6 Release Date.
Rumors surrounding iPhone 6 release date have been rife since after iPhone 5 and 5s. Most of the rumors have consistently pointed to a fall release: September/October 2014. This is the usual time that Apple serves the major upgrade to its iPhones (and in some cases, the iPads too).
Also Read: iPhone 6 Features
That time frame stays. It's June, we've seen iOS 8 now and we know it's coming in fall. Over the weeks, we'll get to know when exactly iOS 8 would be available for the public (it's in the betas now) and that would give us a clear idea about iPhone 6's release date.
Post-iPhone 5s, analysts and industry-watchers have been talking about large-screen iPhones. The earliest rumors about iPhone 6 began with the 4.7″ display feature. We haven't really had any concrete “prototype” leaks that would establish the spec but there have been enough sketches, dummy molds and other related leaks that seem to positively hint at a 4.7″ iPhone 6. There is also the other “bigger” iPhone 6 (the second version) which spans a 5.5″ screen. No further details or leaked images are as yet available of this one.
But for the initial iPhones, almost every iPhone has been announced/released in September or October. It's an yearly-cycle that Apple has stuck to in recent times.
Since every new iPhone ships with the latest iOS firmware that was announced at the WWDC, this year, iPhone 6 will ship with iOS 8. Apple is right now releasing beta versions of iOS 8 and has slated the final, public version for fall.
It is also rumored that along with iPhone 6, Apple could be looking to unveil the smartwatch it has been working on for long. That probability seems very little but it would be great if that happens.