IT'S (BEEN) + DAYS / WEEKS / MONTHS / etc. + SINCE

The structure it's (been) + days / weeks / months / etc. + since is used to emphasise the length of time that has passed since a past event:

It's been weeks since his last blog post.
It's been a long time since I met him.
How long has it been since they moved away?

In the sentences above, it's been stands for it has been (present perfect); however, in everyday speech, it is more common to use the present simple instead of the present perfect:

It's weeks since his last blog post.
It's a long time since I met him.
How long is it since they moved away?