Assuming Apple continues the trend of new generation, refresh, new gen, refresh, etc etc.
My 3-year plan on my 5 is ending this year and I'm kind of waiting on 2016's 7 for a new form factor (or at least a flush camera lens). Even though the battery is a nightmare I'd tough it out for another year.
I like the X cycle, because the new form factor is fun.
But the XS cycle get some amazing new technology sometimes.
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Oh you silly people and your "2-year cycles." I buy the new iPhone EVERY year. Every year I sell my old iPhone for $600+, then I walk into the new one for $150-200 (which is basically the subsidized price when you sign up for 2 years anyway). I am 35 years old. I don't smoke. I don't drink. My personal vice is having the latest and greatest iPhone when it comes out. It costs less and is healthier than those other vices too!
You don't have to justify to us! lol