On top of that, I wonder what the average length of time credit card rewards spend accumulating and not being used, and how much depreciation that represents w.

On top of that, I wonder what the average length of time credit card rewards spend accumulating and not being used, and how much depreciation that represents w.r.t. inflation, not to mention the opportunity cost of having invested it for the same amount of time. And to boot, you pay a yearly fee for those non-interest-bearing funds to sit around!