how long since i bumped into you and your sister at McDonald?

我从http://iask.sina.com.cn/b/15609853.html这里复制的，看对你有没有帮助。

It is +一段时间+since…的句型与It has been +一段时间+since…意思相同。
前者是规范说法，后者是口语说法。

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.

It is +一段时间+since…的句型与It has been +一段时间+since…意思相同。

前者是规范说法，后者是口语说法。

It’s a long time since he studied English
他好久没学英语了。

You haven’t come to see me for two months.
你有两个月没来看我了

其实就是完成时的另一型式

不一样，it has been+段时间 since=it's +段时间（has been 是持续性动作才能加段时间，it's后可直接加段时间）