It's almost official. Apple is expected to send out their "save the date" notices, and that date is September 9. The date is almost exactly a year after the iPhone 5s and iPhone 5c were announced, on September 10, 2013. Last year, online orders began September 13 with availability on the 20. In the case of the gold version of the iPhone 5s, availability was extremely limited.
Assuming Apple's typical schedule, announcing on a Tuesday with availability on the Friday of the next week, the next iPhone would be available ten days after announcement on September 19.
It will be interesting to see if Apple manages their supply chain a little better this time. Now, they know "Gold is best" and ideally will have better availability. What is likely is that an even larger iPhone, with a rumored 5.5-inch screen, will be harder to come by and probably wouldn't be introduced at the same time.
Coincidentally (or perhaps not) Samsung has their own date set, for September 3. That's when they are expected to announce their next tablet, the Galaxy Note 4. So Samsung might squeeze out a few days of press coverage before the industry turns to see what's behind Apple's curtain.