This page summarizes 4,800 words of research on this.

Summary

• just get your hours in
• engaging content sticks better
• worse than a bad system is no system
• 2.5k words / B1 is a general goal for immersion
• you can always achieve faster results by cutting corners or focusing on a subset of language (see military)

Praxis / Actionable Steps

• use a course or frequency vocab list to start out
• find engaging content in TL
• use a system to process new vocab (flashcards / anki, lingq / lwt / lute, etc)
• study grammar

resources

• media you've already consumed, but in TL
• public library / broadcast
• clozemaster.com (theft)
• braille needs physical flashcards

Theory

approximate vocab count to CEFR level equivalence

sources: polydog forum article, outcast, fi12, hsk vocab equivalence

passive vocab collapses in on itself to match active vocab when taking a break (eg B2 -> B1).

this is temporary. recovery is fast.

comprehension percentage could be a good estimate.

but c% and native speaker age / reading grade are very fickle to correlate.

not enough research papers exist on the vocab size of young kids.

approximate learning time

at 10 words/day, getting to 2,500 words will take 250 days.

this is a steep rate (imho) and this doesnt even factor in the reviews you have to do each day, off-days, and standard forget rate.