If the current blockexplorer average interval between blocks is to be believed, then difficulty changes every 12.6 days:
http://blockexplorer.com/q/intervalSo, stochastically.com puts the Div Yield at 4.52%. That's 2.64% for 7 days. By comparison, Gigamining currently has 1.67%, Bitbond 1.81%. Right? I really like this asset, but the ROI calculation is a PITA. Math is hard. I shouldn't do investing. :-)
http://www.stochastically.com