This site shows a general overview for each state. I don't know if it's current or accurate so best to check with law enforcement to be sure but it shows NC to be a "35% Net" state. That means that the total amount of light able to shine thru your windows must be at least 35%. This does not mean that 35% Tint is legal however and in a 35% net state it would not be. The reason? Well, your factory windows are already tinted and usually allow about 80% light thru. If you put 35% tint on top of 80% you are now down to only about 28% light transmission and this would be below the 35% requirement and illegal. Check with a good shop but don't always assume that they know the law, it's your car and you are responsible not them.
Just had mine tinted yesterday. Used 35% tint and I think its too dark for my own taste. You can see out fine, but it looks too dark from outside. I am not worried about any tickets here in NY (Wink, Wink).