I feel like we do know how much liquidity is too much, in the sense that the HFT traders are siphoning profits from value-seekers to rent-seekers. The total of their profits is the amount by which there is too much liquidity.

It's not necessarily clear whether it could be reduced to seconds of liquidity, or minutes, but it's clear that milliseconds is too short. The money isn't remaining in the market long enough to provide value -- especially when the market has already been made and they're just trying to figure out who gets the surplus between ask and bid.