It’s a long time since …… 后加什么时态 是固定的吗 还是看情况

It's a long time since …… 后加什么时态 是固定的吗 还是看情况
这是固定句型吗?

since的四种用法
1) since +过去一个时间点 (如具体的年、月、日期、钟点、1980,last month,half past six).
I have been here since 1989.
2) since +一段时间+ ago
I have been here since five months ago.
3) since +从句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
4) It is +一段时间+ since从句
It is two years since I became a postgraduate student.
你的是第四种情况
固定句型.可以这么说吧,不过算是一种很经典的句型,在我给你的网址里有简单介绍,就是“35种经典句型”.