How often should Apple introduce a new iPhone?

[thermal]

When Apple introduced a new iPhone once per year, some complained that was far too slow in the hyper-paced smartphone world.

Then Apple introduced the iPad 4 about seven months after the iPad 3, and many bemoaned the faster pace of product introductions.

So, my question is: how often should Apple introduce a new, updated iPhone (iPad in separate discussion). Once a year as during Steve Job's era? Twice a year to remain competitive with Android's hyper pace? Some other figure?