Our memory is structured so that we forget things at an exponential rate (exp(-a t)). The better we remember things, the smaller the coefficient (a). You can look around for pictures of "forgetting curves" [1].
The idea is that for new knowledge, the coefficient (a) is large, so you need to refresh your memory sooner. As you retain the knowledge (the coefficient gets smaller), you can wait longer before you need to refresh your memory. The exact coefficient is going to be dependent on the person, the type of knowledge (for the person), the context in which it's learned etc. etc.
A simple way to do this without flash card applications is to try to recall the information you've learned periodically. Can you recall it after a day? A week? A month? If you can't, go back to the information and relearn it, and try again, making sure to do active recall to refresh your memory.
Another trick is to try to explain the knowledge you've learned to someone else, or pretend you're trying to explain it to someone else, if you don't want to subject your friends to it. Often times a "why" will come up or some piece of it will seem arbitrary that you won't be able to explain, which is a good signal that there's a concept you've missed and need to review to understand it more deeply.
[0] https://en.wikipedia.org/wiki/Spaced_repetition
[1] https://elitemedicalprep.com/what-is-spaced-repetition-and-w...