What are SM-2's interval rules?

The first two intervals are fixed, then each interval is the previous one times the ease factor.

$$I_1 = 1 \text{ day}, \quad I_2 = 6 \text{ days}, \quad I_n = I_{n-1} \times EF \ (n \ge 3)$$

So with a typical starting ease of $EF = 2.5$: 1 day → 6 days → 15 days → ~37 days → ~94 days, and so on. Each successful review multiplies the wait, which is what makes the schedule expanding.

An easy card (high EF) shoots out to long intervals fast; a hard card (low EF) crawls. The whole schedule hinges on that one multiplier.

These figures (1 day, 6 days, then ×EF) are SM-2's classic textbook defaults that teach how the algorithm works — not this app's live settings. This app schedules with FSRS, a newer model, so the actual gaps you see between reviews are computed differently.