By years you mean 4 years right?
iPhone 4: June 24, 2010
Apple has no rhyme or reason when they release or announce their products. They release it when it's ready.
Yes, which means the last 3 iPhones (4s, 5, 5s) were announced in September/October. That also means that only 3 iPhones (3G, 3GS, 4) were announced in June/July, which means they release dates are neck and neck. Really, we could see anything this year. Hell, we might not even see a new iPhone this year. We could have to wait until early 2015 for a new iPhone.
Now before someone jumps on my back saying "What about the original iPhone?". The original iPhone was
released in June 2007, but
announced in Jan 2007.
Now yes, Apple has no release cycle set in stone, but you also have to take into account how many iPhones fans have been added in the last few years who are now used to the September/October release date. When the first four iPhones were announced, they were on one carrier from the get-go. Now, we have had 3 newer iPhones on most carriers in the US (all the major ones).
Main point? Apple would be dumb to release the iPhone 3 months before, or 3 months after the Sept/Oct release window. Why? Look at just some of the polls here on MacRumors. Most people who will be upgrading to the 6 will be former 5 owners who's contracts are up in Sept or later. Now, thats not to say the new iPhone won't be successful if they release it early or late, but it would probably be more successful if they release it at the same time they have in the last 3 years.
If they release the iPhone 6 in June, I will probably still upgrade. Its not worth it to me to wait 4 months until my contracts up to get the iPhone, nor to wait another year for the 6s. At that point, my ETF would be $110. Opening a new line of service with my GVoice port will get me a $100 bill credit (if they'll still be running that), so I'll only spend $10 to cancel my contract.