it's been long since

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 between 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 compare 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 answered 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.