I stopped after I realized that buying a higher tier of tax software just to handle multiple income sources ate at least the first $15 of income (which is a significant time investment in Mechanical Turk).
In other words the income was so low that even paying the taxes on it ruined the whole venture. Even Bing's rewards have a higher rate of return. The $2/hour from this article entirely mirrors my experience (although my average might be a little lower).