I spent at least 10,000 hours in the use of SuperMemo, for fifteen years.
Four hours a day, 28 days a month, eleven months a year, for FIFTEEN years, solid.
But there are other SPACED-REPETITION SYSTEMS that have emerged.
I can only talk of what I personally used.
These gentlemen talk of what Spaced-Repetition is about:
“How to Use Spaced Repetition in 3 Minutes” – by Karl Avillo, MD
“The Best Spaced Repetition Learning System”: