Hmm... Having just taking an Abnormal Psychology course, I feel inclined to give an overly detailed DSM-style criteria, such as:

A period of at least three months in which two or more of the following symptoms occur at least once a week, with at least two symptoms involving activity outside of one's own account in any one-month period: submitting a deviation; submitting a journal entry or news article; commenting on a deviation; commenting on a journal entry or news article; participating in a chat or forum discussion. Only one symptom is necessary if it is a comment on a deviation that is particular detailed.

In seriousness, I'm not sure if there is really a quantity issue for activeness, but I do feel that a truly active member has to have some sort of activity in more than one area of deviantART... An active member will also have the desire to be active, even if they lack the time for it. In other words, someone who spends hours every day just killing time online and only comes to dA once or twice a week is not active, but someone who only has the opportunity to be online once or twice a week but always pays dA at least a quick visit is active.