I think they'll do it when they're good and ready. That said, Apple has announced an "enormous" iPhone event starting May 8th. . . the quoted word sounds interesting.
It is a big push to get folks to go ahead and update their previous iPhone models to a 5c or 5s (I believe it is to clear the shelves as much as possible before the next iPhone is announced - which always slows sales of current models).
http://www.cnet.com/news/apple-plans-enormous-iphone-push-this-week-report-says/