On a contract, I like to keep mine for two years although I kept my iPhone 4S for five years as I hated the 6, 6S, 7 design and did not upgrade until the X. The same right no for me in that I love the 5.8" screen and the rumours of the 12 Pro starting at 6.1" has put me off 100%. Right now i'm looking to the 11 Pro although since there are zero deals through the carriers on 2 year contracts, I may buy the phone outright, put in a SIM only contract and then keep the phone for a few years until Apple make a smaller Pro phone. I travel and I'm not a fan of huge phones.
For me I use the iPhone for everything - I'm a tech minimalist and purely use a MacBook and iPhone along with Apple TV. This combination has been perfect for me for the last 10 years now and stress free

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I'm really not excited about the 12 new design rumours. In all honesty, I think Apple nailed it with the X - 11 Pro design.