Question

it's been long since

Answer 1

It’s been a long time since = It has been a long time since Examples : It has been a long time since I visited my grandpa last year. It has been a long time since I heard you sing. It has been a long time since we moved to London . All above are referring to a difference on timing beeen an action / a situation happened in the past. We do not use “It had been a long time” because “It has been a long time” is still correct in above 3 examples in terms of tense. Example on using “It had been a long time” : It had been a long time since I had had a dinner with John. Here we use “had been a long time” to pare the timing with “I had had dinner”. When we have to use past tense such as in a book report or a reported speech
we need to use “had been” to tell people when action happened first and which action happened later. By the way sorry that I wered your question in English. I tried to do this and use simple English so you can understand easier. If we always try to have a Chinese explanation we cannot improve our English reading skills.
参考: Myself