Now, in her defense, I'll note that we ran out of Diet Pepsi regularly under the previous office manager, because she couldn't believe how much of it we drank either. But the answer turns out to be about two cases a week or eight cases a month. We also drink a lot of Diet Mountain Dew. We drink a couple of cases of Caffeine Free Diet Pepsi each month, which I think is more than we drink of Diet Coke.
What we mostly don't drink is sugared soda. At one point, we had eleven cases of Coke waiting to be consumed, because the receptionist / office manager that we had briefly kept ordering more Coke without actually counting what was sitting back there. I managed to get rid of three or four cases while decadent_dave and catalana were in town by trading Diet Pepsi (which we were out of here) for Coke, since our guests drank sugared soda. I think we've still got about seven cases of Coke here.
But we're slowly coming back into balance. Now, if we could just get someone to drink the two cases of Sprite that are lingering on the shelf.