If you remember to make all your repetitions every day, SuperMemo will make sure you remember 95% of the learned material.

You can increase this number at the cost of the speed of learning. You can also learn faster but remember less.
For more details see: Forgetting index at supermemo.com

Rarely, your material may be so difficult that SuperMemo will not be able to guarantee 95% recall. To prevent this from happening read: 20 rules of formulating knowledge at supermemo.com