I have only just signed on to the forum so please excuse the late reply.
I cannot comment on Naja in paticular but I have a three year old tiger that went about three months without feeding, six months ago. I don't know why and he lost very little condition I changed his tank and surroundings and he immediatly started feeding again, don't know if that was coincidence or what. Up until then I don't think he had ever refused a feed but he now seems to eat less and feeds when he is ready,but is still growing nicely and in excellent condition.