About 4 to 5 feet, and I will turn my head 0 degrees. "Head turning" isn't a thing with large/multiple monitors. It's a manufactured problem by people who think multiple monitors are somehow bad or evil or just have never used them.

I have a comfortable chair with a headrest, my head sits against the headrest and my eyeballs do all of the moving, my neck doesn't need to move a millimeter to view the entire area of the screen(s).

Neck strain is simply not a thing with multiple monitors unless you have so many monitors that some sit behind you - but that is not what we are talking about. We're talking about 2 or 3 screens or an ultrawide or a large 55" or 65" curved screen sitting in front of you on a desk. None of these scenarios requires turning your neck at all, unless your eyeballs are permanently fixed dead ahead and you cannot move your eyeballs at all due to some serious neurological problem, and then you should go see a doctor.