The way feasting unhappiness works is this:
One of the potential unhappiness factors is "We cannot forget your cruel oppression" which is ridiculous because they do forget fairly quickly. Possible sources of cruel oppression that they very easily forget is slavery whipping, vampire feasting, and various events - almost anything that says "temporary unhappiness" in its description (not the draft, separate mechanic, IIRC). The number of angry faces you get in the city for temporary unhappiness is directly tied to how long the unhappiness lasts. In a normal speed game (it scales based on game speed) 10 turns of unhappiness = 1 unhappy face. You always round up to the nearest whole number of angry faces, so 1-10 turns of temporary unhappiness = 1 angry face, 11-20 = 2 angry faces 21-30 = 3, etc. If a city currently has temporary unhappiness from one source and you cause more from some other source, it just adds to the number of turns of unhapiness in the city. If this doesn't increase it past the next multiple of 10, then it won't increase the number of angry faces. Using slavery always will because whipping adds 10 every time (regardless of pop lost). Easy way to determine who many turns of temp unhappiness you have left is to mouse over the slavery rush button and subtract 10 from the number of turns it displays.
Feasting causes 3 turns of temporary unhappiness (on normal speed), so if there isn't any temporary unhappiness in the city already, you can feast 3 times in one turn for just one angry face (9 turns of temporary unhappiness). Once 2 turns have passed, it will be down to 7 and you can feast once more without getting any extra unhappy faces. Past that you have to wait 3 turns between feasting if you want to maintain only one angry face from feasting.
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