Almost all the prior literature on the Spacing effect and on naturalistic applications of a Spaced repetition memory system concern declarative information like word pairs, term definitions, or simple property values. I suspect that conceptual information, densely linked to prior and related knowledge, will exhibit very different memory dynamics.
In particular, I expect that much more conservative review schedules can be used, effectively lowering the cost of retaining this type of material: Optimizing spaced repetition system schedules.
Early experiments with Quantum Country exhibit much slower forgetting curves than those reported in the literature, at least for many questions: QCVC questions are initially forgotten at very different rates; Quantum Country users rarely forget after demonstrating five-day retention. This may be in part because of the information is conceptual, interconnected, and introduced within the context of a narrative (The mnemonic medium gives structure to normally-atomized spaced repetition memory prompts). But another explanation: Quantum Country’s high recall accuracy rates may mask forgetting because of cuing.
More on spaced repetition systems and conceptual knowledge: Spaced repetition memory systems can be used to develop conceptual understanding