I want to use this sentence, but I am not sure whether or not it is correct.
My sentence is:
"It has been a long time since we are working on the same topic"
The first part of the sentence is fine, but the second part sounds bad. basically, you're describing a finished action in the past, and a non finished action since then (has been). so the second part should be expressed with the past simple tense:
"It has been a long time since we worked on the same topic".
As usual, note that in the spoken language, "it has been" will be contracted to just "it's been".
One way of writing the sentence would be,
It has been a long time since we have been working on the same topic.
Hope this is what you meant to say and the answer helps!