It is generally a good rule of thumb that cereal should be discarded when it is two years or longer beyond the expiration date, or when it will no longer fall out of the box by itself. Chips, however, never die. No matter how soft they get. It isn't actually poisonous at this stage. If it spits the milk back out at you, it's no longer good.
Milk is spoiled when it starts to look like yogurt.
Yogurt is spoiled when it starts to look like cottage cheese. Cottage cheese is spoiled when it starts to look like regular cheese. Regular cheese is nothing but spoiled milk anyway and can't get any more spoiled than it is already. Most people think this is acceptable. When something starts pecking its way out of the shell, the egg is probably past its prime.