And then how many lemmas do you need to know to understand 75% of text? I see the number 1000 thrown around a lot, but I don't think that's enough; it might not even be enough for 50%.

That 75% number is vocab coverage, not comprehension.

You should be able to learn 50 per day. I used to do flashcards before using lingq and it took me about 1.5 hours to commit 50 words to active memory. If you use flashcards then it's important to break it into at least 4-5 different sessions per day. To achieve this same pace using lingq it would probably take at least 3-4 hours per day (I've done that as well). theoretically, 6 months would be enough time to reach your goal (9000 lemmas), but you'd probably have to schedule some sanity breaks in there. so figure 7-9 months of dedicated practice. This would be just for reading. Listening and speaking would take years to reach the same level.

Speaking very roughly the following may be a useful way to think about European languages.

An advanced speaker is likely to know/use/recognise around 500 different verbs and perhaps the same number of function words (conjuctions, prepositions, adverbs of place/time, etc). In terms of nouns the number is much higher (in the order of ten times larger I imagine). For simplicity let's say that a similar number of adjectives would be nice (this is probably a bit excessive).

The difficulty lies in deciding how many nouns are needed as this is very context sensitve (linguists insist on talking about 'fields'). For example, you can be an advanced learner of Englihs without knowing any of the myriad of terms used to describe cricket (or sport in general). As a native speaker, there are loads of cricket-words that I don't understand despite some people using them all the time. The number of nouns is sometimes quoted as around 5000, although you could either double or half that depending 'who you ask'.

You can probably get a reasonable understanding of many texts by halving the numbers above: 250 verbs, 250 function words, 2500 nouns and a few other things is a very good start in any language.

In terms of how many lemmas you could learn in a day (let's assume that they have no obvious link to any language you already speak, or you could just find a list of 1000s of cognates for many languages) you could certainly try to learn more than 10 but probably less than 100. (Sorry if that is rather vague!) The difficulty with any kind of SRS approach is that rather quickly the vast majority of your time will be spent reviewing words you have already encounted.

When you know a 1,000 words, this is what reading a native-written text will be like: "Now let's go to the X, after we can X, and maybe later we should X, Y, Z before going to X on our way home." X, Y, Z = words you don't know. This is basically what it means to understand "75% of the text" - you might understand 75% of the words, but you really won't understand the meaning of most sentences, because the words that carry the most meaning are beyond that 1,000 word goal.

My advice for you is to ignore these silly concepts of "just learn the most common 1,000" (or 2,000, or 5,000) words, and just start reading and listening every day. If you set a goal of creating 100 LingQs everyday for the next 90 days, you'll see a huge increase in your comprehension and ability to use the language at the end of those three months. If you set a goal of reading and listening for a half hour or an hour every day - same thing, you'll see a huge jump in your abilities. Focus more on "process goals" (Q: What should I accomplish today? A: Read and listen for 30+ minutes) and less on getting to know some arbitrary number of words.

What's the average for words in one lemma in English? I guess it's safe to say the average is about 4 (rather give than take). After I've reached the 12000 words point on LingQ (mostly without apostrophes and proper nouns) I understood at the very least 95% of virtually any (written) text. Which means roughly 4000 of lemmas are enough for that.

Not that anybody asked me, but I think for the purpose of learning, lemmas are an impractical concept. It's a linguistic term, and as all the linguistic terms are, it is a meta- language for describing features of the language. So why bother if for now you're just learning the basics of the language and not going to become a linguist (in the academical sense)?