The gym I go to now doesn't have a neck machine and it pisses me off.
It's funny when I was only 200 pounds or so I was training my neck once a week, my neck was 20 inches. I got asked if I was a pro fighter or boxer all the time. Way heavier these days and leaner,even my traps and shoulders have grown in spades... but I stopped training my neck directly and it's only about 18.5 now. I get asked do you compete" a lot more but never really get asked if Im a fighter.
Your neck should be at least 17 inches...18-20 is ideal. Anything bigger is stupid IMO