Apple will usually have a major refresh of its iPhone every two years and a minor refresh every year. Such as in the past, an iPhone 3G followed by the iPhone 3GS and iPhone 4 followed by iPhone 4S a year later.
Apparently, after having revealed the iPhone 5 in Fall this year, the Cupertino company is expected to unveil the iPhone 5S sometimes next year. And rumors suggest that it could be less than a year that Apple would release the iPhone 5S.
According to Digitimes who relayed a report from Commercial Times, Apple is very likely to start trial production of the iPhone 5S as early as this year. If the trial production is successful the volume production could start as early as in Q1 2013 which would probably bring the release of the iPhone 5′s successor earlier than usual.
Anyway, this is currently just a rumor which you should just take it as a grain of salt. But it wouldn’t be too surprising as Apple did refresh its iPad less than a year.