I used the FSRS simulator (as I mentioned previously) to simulate 100 years of Anki with a collection that has 30 thousand cards. Desired retention=0.9.
maximumInterval = 365 # only this setting was changed between simulations
new_cards_limits = 10
review_limits = 9999
max_time_limts = 10000
learn_days = 36500
deck_size = 30_000
Here are the results:
However, I think it’s easier to interpret if I just report average total time per card:
Average total time per card is basically “how many minutes you will spend on this card over it’s entire lifetime”.
It’s worth mentioning that for max. intervals of 100 years and 10 years, retention accurately matched desired retention (in FSRS), but for 5 years and 1 year retention exceeded desired retention.
Overall, I think 10 years is a reasonable max. interval.