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".