What is the "temporal ridgeline" from Cepeda et al. (2008), and what gap should you use?

The optimal GAP between sessions grows with how long you must retain, but the gap-to-retention RATIO shrinks — roughly 10–20% of your target interval for short goals, dropping toward ~5% for a one-year goal.

Cepeda and colleagues mapped how the ideal study gap depends on the target retention interval. Two facts fall out. First, the longer you need to remember something, the longer the gap between sessions should be. Second, the ideal ratio of gap to target shrinks as the target grows. Practical rules of thumb: to remember for about a week, space by roughly a day; to remember for a year, space by weeks to months. There's no single magic interval — it scales with your goal, and this is well-replicated.