You are looking at a complicated, question, but one which research can answer. In seeking an answer, some ambiguities in the way you framed your question need to be addressed:
There are some other issues in your question that are not readily apparent:
- The Term "should" implies that there have been some currently determined standards against which a given speaker can be compared.
- What is an "Average Endlish (sic) speaker?" Is he/she an adult? Is English the speaker's native language. What about the level of educational attainment of this speaker? What social, occupational, or environmental factors determine the vocabularly used by this speaker.
- You need to define what you mean by the term "Active Vocabulary."
- What does it mean to "...know in general?" Does this refer to recognition of a word, being able to use it in a sentence, being able to provide a formal definition?
To get some assistance in pursuing your question, you may want to look at this site: Companion Website for: Word Frequencies in Written and Spoken English
- I wonder why you have selected spoken English, rather than written. Variability of usage frequency is so much dependent on the context of the communicaton wth respect to both speaker and audience, that you would have to record all words used by a representative random sample of speakers having the attributes described above, within a variety of audience contexts, to determine the frequency of usage for any given word.
- You would be far better off looking at word frequencies as used in school text books at grade levles 1-12, to get a feel for the most commonly used words at those grade levles.