I'm guessing an August-September time frame for a couple reasons...
1 - near the beginning of school/college
2 - good timing for Christmas sales
3 - six months after the expected March 7th launch of iPad 3 and (maybe) iOS 5.1
4 - good time for iOS6 launch, same reason as #3.
The late-in-the-year debut of the iPhone 4S last year really crowded the Christmas sales season. Surprisingly, it was still a huge success, but I'm wondering how many iPhone 4 people upgraded. I'm thinking most of the record sales were by people new to iPhone - especially as the Android market is increasingly glutted with devices carrying older operating systems, and many plagued with major issues.
However, those of us waiting for an iPhone 5 June launch last year, then July, then August, then Sept, etc. were terribly
frustrated when an iPhone 4-remake materialized. I can't help but think there were other factors involved in the delayed launch of the 4S and the absence of a iPhone 5 - not the least of which could have been the health of Mr. Jobs.
Although I think it will be in the 3rd quarter, I can't help but hope it will be June. I'm still with my iPhone 4 (and happy with it on iOS5), but plan to upgrade - IF it's a significant improvement.