Skyler once asked Walt "How much is enough? How big does this pile have to be?". When Walter White makes the decision to begin his money-making venture it's for the noble reason of ensuring his wife and children are taken care of when or if his newly diagnosed cancer finally takes his life. Although he does not explicitly state a target amount he wishes to earn, it's clear that Walt does not envisage he would end up making the kinds of money he ends up making. Hell, for most of the show he fails to hang on to whatever cash he's earned because he needs to pay for his treatment and later his brother-in-law's recovery after he is shot. It isn't clear exactly when Walt became aware of his earning potential but if I were to hazard a guess I would say it was when he met fried chicken kingpin Gustavo Fring. The seemingly easy life Fring lives despite being a high profile drug lord appeals to Walt and when he eventually removes Fring from the equation it's clear he intends to fill his former employers shoes. Walt's greed does decline somewhat towards the end of season 5b and he even forgoes finding where the remainder of his 80 million dollar fortune has been hidden just so he can put a bullet in the head of the man who killed his brother-in-law. In fact by the final episode Walt has returned to his original ethos that as long as there is enough money for his son and daughter - and maybe Skyler - to be comfortable for the remainder of their lives then his actions have not been for nothing.
