Each radical you guru will unlock further lessons.
Specifically, the SRS intervals look like this, where the time is halved for level 1 and 2:
Fastest progress possible| Review | SRS Level | Total Wait | Next Level |
|---|---|---|---|
| Lesson | Apprentice 1 | - | 4h |
| 1 | Apprentice 2 | 4h | 8h |
| 2 | Apprentice 3 | 12h | ~1d |
| 3 | Apprentice 4 | 1d 11h | ~2d |
| 4 | Guru 1 | 3d 10h | ~1w |
| 5 | Guru 2 | 1w 3d 9h | ~2w |
| 6 | Master | 3w 3d 8h | ~1M |
| 7 | Enlightened | 7w 5d 7h | ~4M |
| 8 | Burned | 24w 6d 6h | - |