If I understood it the first time, it usually sticks very well. I'm refraining from saying completely and always because it's obviously not an absolute.

What I meant with that last sentence was that even for material I did not understand well (because it seemed to go over my head), I usually still finish reading the material, trying to absorb a very handwavey gist or general structure of the idea, avoiding the need for closure and leaving large chunks unclear. Then, after several days, I often find that I now almost magically understand it because it just clicked without me even being aware of it.