I have long been looking for you/ l have been looking for you for long. Is their any difference in meaning?
"I have long been looking for you," is correct.
"I have been looking for you for long," is not correct.
"Long" is used as an adverb in the first example. In the second example, you are attempting to use long as the object of the preposition, and you can't, because it is not a noun.
That said, "I have long been looking for you," sounds a bit flowery and proper. Most people in my part of the world would say:
I have been looking for you for a long time.
We do often use "for long" as an abbreviated version of "for a long time:"
He hasn't been here for long.
However, that idiom only works in some sentence constructions. It's odd. You could ask this question:
Have you been looking for me for long?
But you can't rephrase that as a statement. It seems to work only in negative and interrogative.