The quotation is inaccurate, or inaccurately attributed.
In The Marriage of Heaven and Hell, Blake includes four sets of “Proverbs of Hell”, each “proverb” printed out (by Blake himself, in the case of this book) as its own line.
The third “proverb” of the first batch is:
The road of excess leads to the palace of wisdom.
“Excess” is not the same as “wisdom”, whose “palace” this wording suggests might consist of prudence or moderation, and whose “palace” might be reached by other “road[s]”.
The seventh “proverb” of the third batch is:
You never know what is enough unless you know what is more than enough.
–which anticipates one of Hegel’s Big Ideas, namely, that when one comprehends a boundary in any sense, one has conceptually crossed or is straddling it.
For a concrete, if vulgar, example, if you really understand how much alcohol to drink is too much to drink, then, conceptually, you already have a foot on the ‘somehow unpleasantly drunk’ side of however much there is to drink.
A more conventionally philosophical realization of this Big Idea (of boundaries) is this: if one comprehends Being in the sense of comprehending the negation of negation, one has, entailed in one’s comprehension of the Concept, already the comprehension of Becoming (if one is willing to work for it).
wikipedia has a useful blogicle which puts The Marriage a) in the context a critique of Swedenborg and a seemingly impermeable division of ‘heaven = spirit’ from ‘hell = body’, and b) contemporary discourses (and action) of political emancipation by way of revolution: http://en.wikipedia.org/wiki/The_Marriage_of_Heaven_and_Hell . This blogicle doesn’t scruple to mention naughty Bataille.