Cardsystem 9 weeks

Its like a calendar: Every slot represents one day. Sundays in red mark the end of each week. All together they add up to 4 weeks or 28 days.

The easier a card is remembered
The further it is put into the future and therefore requires less often.

This principle leads to a maximal perfected tool for learning.

In my opinion the 9 weeks version would be the best option for most students.