I doubt that any mother would be able to provide enough breast milk to fuel an adult, even if one could survive on it: However, If you're talking about stored milk from multiple sources, then I'm not sure if it could sustain an adult.
there is an account of a lady who kept a a stranded boat of 16 people alive on breastmilk for 2 weeks!
http://www.breastfeeding.co.uk/articles/article020 .html
an
adult couldn't survive mainly because after a year old or so you do need vitamins/minerals in greater amounts. there is enough iron (despite what follow on milk companies would have you believe) for example for a baby under a year but not enough for adults. Also if you were sedentary i'd assume you'd have no exposure to sunlight and therefore would be deficient in vitamin d anyway!
It certainly didnt work for the pope. Alexander the sixth, one of those popes they dont hang many pictures of, was given mothers milk to help him get over being accidentally poisoned by his idiot, disease ridden son. it didnt work.