12 month cycle
here's why:
I like to feel like my XYZ is "new" for a reasonable amount of time. Why? Because I'm a geek and for some things, I just like having "new". I'll buy not-new stuff. From clothing (goodwill/salvation army represent) to houses, some things I just don't care if it's not new. But because I'm somewhat of a gadget-geek (I like cars too), in that realm, I want to feel like I've got something new. Call it pretentious, shallow, whatever... I bet there are things about everyone that could be deemed that way. Anyway, yes, for the iPhone, I like to feel like it's not out-dated the minute it comes out.
But why is that? Well, I think subconsiously, it's a lot of factors. Think about automobile cycles. A typical cycle is usually 4-6 years. Any longer and consumers get bored of it, see the company as stagnant, desire something new.... but have you wondered what consumers would think if the MY lifecycle got shorter? I personally think as a consumer I would be a bit put-off by a shorter lifecycle (car or iphone). I just feel like if a company pumps things out too quickly there's a (mis?)conception that devalues something. Maybe it's the conception that if something comes out really frequently that due dilligence wasn't done. R&D and planning couldn't have been carried out properly, etc.
Sort of the same thought behind pricing. If something is cheap at walmart, it's seen as such. A $20 toaster at walmart is seen as a cheap item. Put the same exact toaster (same brand or otherwise) at an appilance store for $35 and there's a perception that somehow it's a better product.
Interestingly, when the aforementioned toaster is at a high-end appliance store and the price is cut to $15 from $35, people ALSO have perceptions of that...
I think the same type of principles apply to smartphone release cycles. Whether they're genuine concepts or just misconceptions -- it just feels better as a consumer who values "newness" in gadgetry when the product release cycles are longer.