If the language is newer then I tend to repeat lessons many times. The more familiar I get with it, the less frequently I repeat content.
My spreadsheet tells me I’m a little over 70 hours into Russian (some on LingQ, some before I found LingQ–Duolingo, grammar workbook, teach yourself book), and I need to repeat lessons sometimes dozens of times to get the gist of them through listening. It is (slowly) getting easier. Being able to retrieve what I’ve read or heard takes many more repetitions, though I don’t always take it that far. Like most people here I think a large amount of input is generally the way to go…once you’ve gotten to the point the basic sounds and words start to stick a little bit in your head. This can take a long time (Russian, Georgian), a moderate amount of time (Arabic, Irish), or it can happen right away or very quickly (Portuguese, Danish). Portuguese for example I’ve only studied a little over 26 hours, but the words stick very easily and I don’t need to repeat lessons much.
After I feel that I can start to remember at least a little of the lesson, then I reduce the amount of repetition and move on to new material sooner. At some vague point later I mostly stop repeating and move on when I’ve understood a majority of the lesson. This is where I’m at in French and Norwegian and the words just start piling up with little effort while reading. Listening still requires some repetition sometimes, but I sense that its not too far off where I won’t need much repetition there either.
Edit: Reflections on my Arabic experience.
Arabic is usually considered very difficult in general to learn and pronounce. Many years ago, I took a full year of the language in college. We spent most of the first month working on pronunciation. Because of that effort, the pronunciation doesn’t feel difficult, even with 15 years of completely ignoring the language. Since the sounds are familiar, new words tend to stick with only a moderate amount of repetition.
Contrast this with Russian or Georgian. Most of the individual sounds in Russian are not difficult, but the combinations are. Georgian has both difficult individual sounds and sound combinations. In both of these languages it is very hard to remember words, even after repeating them many, many times.
The lesson I think is this: If the sounds (letters and combinations of letters) are familiar, the brain seems to remember words and phrases that use them easier than if the sounds are unfamiliar (I guess that’s obvious). Until the sounds are familiar, more energy is spent on trying to decipher the individual sounds and there is little leftover for connecting the sound patterns to meaning patterns. So more time must be spent on repetition early on. This is why I think its a good idea in the beginning to focus on pronunciation issues. Not for speaking necessarily, but to sooner reach the stage where input becomes optimal.