I believe the answer to this problem is the same as your vintage data question plus a couple of steps.
You need to reconfigure your data such that you have 1 row for each card, a column for card acquired date/time and card registered date/time, and you should be able to write a calculated field something like:
Name: Time Until Card is Registered
DateDiff('second',[Card Registered],[Card Acquired])
Assuming each card only has one registration and activation date you can do a Window_AVG(sum([Time Until Card is Registered])) and you would need to 'compute using' some kind of card ID, I believe.
I still don't have a firm enough grip on Table Calculations to say that with confidence, especially without seeing a sample workbook.
As a note, you get better and more answers when you attach a workbook with your problem.
If you're concerned about confidentiality I recommend creating a set of dummy data, even if it's small.
I hope this helps!