“For how long have you been…” vs. “how long have you been…”

Question

Ante-scriptum: The question should be quite a frequently arising one, so this might be a duplicate. If it is, I haven't found it previously asked here

I don't know if the title is meaningful, but here's the question, anyway. What I want to know is whether the for at the beginning of the following question should be removed or not:

For how long have you been eating?

I consider the sentence to be correct, but some days ago, a teacher (not mine) told me that it was not. And then I remembered that someone else had told me that while it is OK to use the for, the tendency is to get rid of it.

So what's your take on it?

Answer 1

The Corpus of Contemporary American English has only one occurrence of “For how long have you been”, but many occurrences of “how long have you been” used in this sense. So, it seems that, at least in American English, for is not used. The numbers are such that I'd say it probably holds in other dialects.

Answer 2

How long itself expresses duration. So it may not be necessary to use to use 'for' in the beginning of the sentence.

Answer 3

Definitely "For how long ..." is not common and is likely "old English", much like "With whom ....". It follows the same pattern as others, where the word 'for', 'with', etc are placed at the end of the sentence. For example: How long have you been waiting for? ; How long have you been living here (for)? ; Who are you going to the concert with?