Is there a "Total Bitcoin Days" metric?
I think you are viewing the metric incorrectly. The notion of Bitcoin Days Destroyed is not relative to any other standard of measure. Rather, it is the standard of measure.

To clarify, there is no "gold standard" for Bitcoin Days Destroyed saying "if all Bitcoins had been spent within X weeks the total would be Y". That just doesn't make any sense. Think about it like this: how would one calculate what the DOW would be at today, had it not fallen 200 points? One wouldn't! It cannot be done; the points are there only to show the relationship of the market from day-to-day, month-to-month, or n-to-n. They have no standard to which they are measured.