Bhardwaj told us that the plan is for them to come at a regular cadence of one major code update per year, with other, incrementalupdates for specific devices sprinkled in as needed.
Whereas the iPhone 3G, the iPhone 4, and the iPhone 5 were the major upgrade ticks, the 3GS and 4S sandwiched in the intervening years provided the more incremental tock updates.