In a large internet-based study led by Nicholas Cepeda in 2008, researchers found that the optimal amount of spacing for long-term retention was approximately a 1:10 ratio.5 For example, in order for research participants to remember information for 30 days, it was optimal to space retrieval every 3 days; in order for participants to remember information for 200 days, it was optimal to space retrieval every 20 days.
