5 years on my 12v BXT-67R. Works OK I think. ................. Reading this thread, maybe one should replace at 5 years regardless, just to avoid being caught. It's $135 at some auto parts stores, so the expensive dealership isn't the only option.
That same model of 12v battery lasted 3 years on a Ford C-Max I had recently, so I'm guessing a 5 year old one in the Focus is about ready for lead recycling.
I remember searching for a replacement BXT-67R-equivalent, and it seemed the Motorcraft one was as cheap or cheaper than a Duralast or something else, so I did replace with a Motorcraft. Comfortable with that, since I didn't know how "sensitive" the C-Max Hybrid system would be to some other brand with possibly different capacity. (An '08 BMW 530xi I had 3 years ago was weirdly incredibly sensitive to watt-hours capacity and internal resistance variations.)