Given the power law distribution of participation in online forums (say, 1% produce comments or moderate, 3% vote, 96% only consume), I wouldn't be surprised to see the threshold have to drop quite a bit (especially based on time of day) in order to keep the wheels turning and to overcome the drop off after the honeymoon period is over and the novelty of dealing with pending comments wears off.

Because of this power law distribution, it may not necessarily turn out to be that bad, since at any given time there may only be 3-4 comments pending for every eligible reviewer.