"Every perception is to some degree an act of creation, and every act of memory is to some degree an act of imagination."
-- Gerald Edelman, Second Nature: Brain Science and Human Knowledge
1 Appendix 1: analysis of Anki study timeAppendix 2: Using Anki to learn APIsAppendix 1: analysis of Anki study timeAugmenting Long-term Memory2020-09-24Journal
If we assume the mean time between failures is 4 years, then over 20 years that means 5 failures, and reviewing 5 failures \* 10 reviews per period = 50 times, for a total of 50 \* 8 seconds = 400 seconds, or about 7 minutes.
If instead we assume the mean time between failures is 7 years, then over 20 years that means roughly 3 failures, and reviewing 3 failures \* 11 reviews per period = 33 times, for a total of 33 \* 8 seconds ≈ 260 seconds, or about 4 minutes.
Note that in Anki's model a failure resets the review interval back to 10 minutes, then to 1 day, 2.4 days, and so on. In practice, that seems much too conservative. After one or two failures with a card I usually catch on, and it would be better if Anki wasn't so draconian in resetting the review schedule. A better review schedule would reduce the total study time, and I wouldn't be surprised if a typical commitment of ˜2 minutes was possible.