I expect they'll be releasing new iPhones every October, maybe September if they can get them out in time, but probably no later than October. It makes good business sense since it's before the holidays with enough lead-time they can ramp up sufficient production, and it gives them enough time between releases that they can still sell plenty of the old model (the 4 was still selling like crazy even just recently). They'll release iPads opposite iPhones in the year.
The next one will be the iPhone 6, since it's the sixth iPhone.