When retrieval practice is spaced, allowing some forgetting to occur between tests, it leads to stronger long-term retention than when it is massed.
The advantage of spaced practice was first observed by Ebbinghaus in 1885 and no study has ever been able to refute it.
Delaying subsequent retrieval practice is more potent for reinforcing retention than immediate practice, because delayed retrieval requires more effort. It appears that the more effortful retrieval is, the more benefit is obtained.
The optimal elapsed time between two retrieval practice sessions has not been determined. It is clear that the elapsed time should increase with the passage of time, but the actual time is unclear.
In the mid-1970s Dr. M. Poage and I designed and operated a remedial math program which served about 3000 students per semester. We were aware of the effect of spaced interruptions of the forgetting curve and considered it in the design and implementation of the program.