Can you use "långt" instead of "länge"?
Nope, since adverb långt is about distance, and länge is about time.
Short and concise answer - thank you! :)
I am not a native English speaker but shouldn't it be more correct to say "Have you been married for long?"?
Both work and are accepted. :)
Does this mean that they are still married or are they divorced now?
Past perfect continues tense in English is used when the process is continuing. So no, they are not divorced. According to English.
How about Swedish tense?
It's the same in Swedish.