Apple is likely to continue its annual update cycle when it launches the iPhone 6s. Despite the occasional rumor of a summer release, most reports put the iPhone 6s release date in fall 2015.
Since moving to the fall iPhone launch schedule, dates have been as follows:
iPhone 4S: Friday October 14, 2011
iPhone 5: Friday September 21, 2012
iPhone 5s: Friday September 20, 2013
iPhone 6: Friday September 19, 2014
The most similar date this year points to a Friday September 18, 2015 launch of the iPhone 6s.