Kinda depends on you. I’m not saying this to be unhelpful, but different people have different retention rates, and even the way we behave and respond when we look things up often differs from person to person. I literally do exactly what you’ve just described: I look things up while reading/watching stuff (newspaper articles, anime etc.), think about them until I understand, then move on. I don’t use flashcards or any SRS (never have for learning languages, frankly), and the closest thing I’ve done to SRS reviews in recent years is using the Kotoba Bot quiz function (all pre-made decks that I only do on completely random days when I feel like rapidly working through words I don’t know too well as a personal challenge or for the sake of expanding my passive vocabulary). Literally nothing I consume goes into anything I look at again later on. Anything I write down is just for the fun of doing some calligraphy + retention benefits in the moment. I don’t review those notes later on; I scribble more random stuff or calculations on those sheets, and at some point, I throw them away. Not that it really means much, but for what it’s worth, I passed the N1 in July this year, so at least some of that knowledge stuck, including stuff that doesn’t come from Chinese (one of my native languages).
However, so here are two important questions:
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How often are you going to consume Japanese content, and how varied is it going to be? The reason this matters is that if you’re not using an SRS, you’re going to have to use something else to strengthen your memory. You won’t remember everything the first time around, so it’s helpful if you get to see the same words again in what you’re consuming (some examples of how to do this: reading news articles about the same themes/types of events, reading a story with recurring characters and a consistent setting, watching an anime that constantly builds on past developments in the story world). In other words, it’s probably going to help you if you pick Japanese content that’s fairly focused initially so that natural repetition can replace SRS.
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How do you look things up? If all you do is glance at a translation and go, I’m going to guarantee that you won’t remember much in the long run. Make sure you understand a bare minimum about the word before closing the definition. Some possible approaches to that:
- How do the kanji in this compound add up to give me the overall meaning?
- Does this word sound similar to another Japanese word I know with a similar meaning? Is there some pattern I should be seeing here? (E.g. のがす and にげる seem unrelated at first glance, but when you realise 逃 is the kanji used for both and they both are spelt with a N_G_[kana] pattern, you might start to wonder if they’re linked, especially since the first means ‘to allow to escape’, and the second means ‘to escape’)
You should also ask yourself how you’ll remember what you’ve just learnt, possibly by creating a mnemonic, or more generally just by finding some way to link that word to what it means. Think about what might make it more memorable, or where you’ve seen it (or a similar concept) before elsewhere. In short, don’t ‘touch and go’ too much. Make at least an effort to remember: if it turns out you’ve forgotten when you next see the word, it’s OK, because you’ll probably still feel like it’s familiar, and you’ll be able to build on that awareness of having forgotten.
Final thought: try to pick something that really interests you, or even that will have you experiencing strong emotions (excitement, sorrow, admiration etc.). The reason is this: if you remember the context in which you learnt a word, it’ll come back to you much more easily later on. Here are some examples from my experience with The Rising of the Shield Hero. I learnt
- 激しい（はげしい）= ‘violent, intense’ in its Classical Japanese form – はげしき – because that’s how it appeared: it was used in a magical spell meant to create a tornado
- 属性（ぞくせい）= ‘attribute’ was something I learnt because of the discussion of magic types, and because of a song sung by one of the characters (probably from Season 1 Episode 7)
- 病（やまい）= ‘illness’, 患う（わずらう）= ‘to be afflicted by’ and 貴重（きちょう）= ‘valuable’ because they were words used in a passionate speech made by one of the characters in defence of the protagonist (Season 1 Episode 4 – this is a speech I’ve listened to over and over again, so I’m completely certain of which episode it’s from)
Another example: while watching Quintessential Quintuplets Season 1, I noted the word ぱくぱく=‘in the manner of munching; voraciously’ on a whiteboard in (I believe) red marker ink and added teeth to the くs in order to symbolise the enthusiastic eating it expressed.
It’s because I remember the context in which I learnt these words – and in essence, how I learnt them – that they stay with me, and I only rarely (if ever) need to look up what they mean a second time in order to remember them permanently. If you can do the same by picking content you’re passionate about, you won’t need an SRS to help you very much.