I'm a teacher, but not of English. English is my first language.
I don't think "as long as" can be used interchangeably with "as far as."
AS LONG AS
"As long as" suggests a duration in time
As long as I have known him, he has always been honest.
During the time I have known him, he has always been honest.
"As long as" also means "because" or "since"
As long as you're going to the store, pick me up a loaf of bread.
Since you're going to the store, pick me up a loaf of bread.
"As long as" also means "if" or "only if"
I can usually finish my homework as long as I start early.
I can usually finish my homework if I start early.
AS FAR AS
On the other hand, "as far as" means "to the degree" or "to the extent"
As far as I know, he has always been honest.
To the extent that I know, he has always been honest.
As far as I can predict, I will be able to finish this homework.
To the degree that I can predict, I will be able to finish this homework.

English Teacher
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