Quantum sufficit is a Late Latin phrase meaning 'to a sufficient extent'; 'moderately'; 'only as much as required'. It seems to have appeared around 1565 to refer specifically to medical doses, and to have extended its scope among the educated set from there. In the mid-1700s it became common to shorten it to quantum suff among the English upper classes, falling out of fashion around 1900. Unsurprisingly, at this point it appears to have been used primarily in reference to having had enough to drink, although it was also used to refer to anything that was becoming a bit of a bore.
This should not be confused with the term quantum satis, which is currently used in medical contexts in a closely related sense.