I don't think they have a defined cycle.  The iFly760 is likely just a number someone (perhaps me) assigned to the future model.  The original was an iFly700, next was the iFly720, next iFly740/iFly740b.  It just seems logical the next would be the 760.  It seems they were at least 2 years apart, perhaps more.
It seems they upgrade the platform when a useful feature becomes available.  As I recall, the 700 didn't have WiFi, The 720 did, neither had an internal battery, and the 740/740b does. 
It's anyone's guess what the 760 will have:  Bigger capacity battery?  Bluetooth?  XM weather capability?  Faster processor?
I'm just as curious as you.