If you take a look at popular discussion boards, you will probably find lots of debates about reading and its usefulness in regard to learning foreign languages. As it turns out, this question fascinates not only language learners but also serious scientists. In 2014, Paul Nation from Victoria University of Wellington (New Zealand) published a paper on this very topic.
In his research Paul tried to answer the following questions:
- Is it possible to acquire decent vocabulary only through reading?
- How much do you need to read to expand your vocabulary by 1000 words every year?
- What is more efficient - unsimplified texts or graded readers?
- How many words do you need to be able to read unsimplified texts?
The first question, as it turns out, is a source of heated debates between researchers. Some of them claim that it's impossible to reach a decent command of a foreign language by relying only on reading, because unsimplified texts are too complicated for most learners. Some, however, point out that there are a lot of graded readers available to English learners of all levels. Paul Nation argues that there is some truth in both parties' arguments but contends that it could be possible to progress relying solely on graded readers.
The next question arises: how hard could it be, or more specifically, how much time learners need to spend reading? In his research, Paul sorts all the words by their frequency and groups them into blocks. The first block contains 1000 of the most frequently-used words, the second block the next thousand and so on. Not surprisingly, it's easier to learn the first block than the second one, and the second is easier than the third. His research shows that in order to learn the words from the first block, students need to spend on reading only 7 minutes per day. Obviously, this is easily achievable. However, further advancements require putting significantly more effort. So, in order to learn the 5th 1000, the student will have to spend on reading at least half an hour per day (or about 3 hours per week). This is a substantial load but it's still feasible. Finally, for the 9th 1000 words, students will have to devote almost 2 hours to reading daily.
It's worth mentioning that Paul Nation's research assumes that students always read books that match their level. Today there are dozens of graded readers, so this is hardly a problem. However, many students want to start reading unsimplified texts as soon as possible and regard reading Tolkien's flicks as something to brag about. Is it really expedient? Paul responds with a decisive "no" and gives the following reasons. First, for a student having a 2000-word vocabulary every 8th word will be unfamiliar. This translates into one unknown word per line, which is way too many. Second, many low-frequency words appear only twice in the entire book, which makes memorizing them a truly formidable task. This makes sense, because the authors of original works rarely view their novels as textbook material for foreign students.
By now it should be quite obvious that a vocabulary of 2000 words is definitely not enough for reading unsimplified texts. What's the required threshold then? Even with a vocabulary of 5000 words, every 22nd word will be unfamiliar, and it's still a great burden. The study estimates the necessary vocabulary at around 9000 words (a decent advanced level or C1 in CEFR terms). Having such a vocabulary allows students to understand 98% of all encountered words. In this case, rare exceptions could be easily guessed from the context or looked up in the dictionary. In fact, the students with such a vocabulary often view the foreign language as their second "mother tongue" and don't require assistance from graded readers.
Let's sum everything up with a couple of practical suggestions. Up until the intermediate (B1) level, using graded readers as a addition to the main coursebook is an excellent idea. Of course, it's advisable to write down unfamiliar words and phrases to your vocabulary notebook. Making use of space-repetition software like Anki or Overfluent at this stage is not strictly speaking necessary, but it could be useful. From "upper-intermediate" to "advanced", it's better to switch to regular textbooks as the main source of new vocabulary. Since further study involves memorizing relatively low-frequency words, making use of SRS programs is highly recommended. After reaching the advanced level, you should start reading unsimplified texts but again, using your personal vocabulary and utilizing SRS programs along the way.
The original study - How much input do you need to learn the most frequent 9,000 words?.