It is +一段时间+since+从句 和 before

It is +一段时间+since...意思是“自......发生以来已经一段时间了”。
如：
it is three years since he died.(自)他去世以来已经三年了。
It is +一段时间+before...意思是“过了一段时间...才发生”
如：
It is 3 years before they accept the fact.过了三年他们才接受这个事实。

since是在...之后，从..以后/以来

before 是在...之前

你说的那两个词都是在句中做连词

It is a long time before the building was accomplished.
这个建筑花了很长时间才建立起来

It has been a long time since Susan said that she want to give up smoking.
离Susan表示她想戒烟已经过了很久了

如果用before,谓语应该用过去时。而since一般用现在完成时。前者强调过去怎样，后者强调过去的一段时间多现在造成的影响。
before:在，，，之前
since:自从，，，

it is 5 days since i left the hospital.之后
it is 5 days before i left the hospital.之前