Given a stability value and desired retention, calculates how many days until the card should be reviewed.
fsrs_interval(stability, desired_retention = 0.9, decay = 0.5)Interval in days until next review
Stability (S) in days
Target retention rate (default 0.9 = 90%)
Decay parameter (w20). Default 0.5 is FSRS-4.5 value.
Derived from the retrievability formula by solving for t: $$I(r, S) = S / factor \cdot (r^{-1/decay} - 1)$$