We don't know how long it takes to do x 2.

我们不知道要多长时间。

麻省理工公开课 - 计算机科学及编程导论课程节选

But I don't really know about how long it would take people to hack, like, the general firewall

但我不知道人们具体要用多长时间才能穿过防火墙，

理想是当密码专家 - SpeakingMax英语口语达人

What's important about the construction of St. Petersburg-- I don't know how many of you have ever been there; I haven't been there for a very, very long time-- this city is not like Moscow at all.

那么修建圣彼得堡到底有什么重要的意义呢,我不知道你们之中有多少人去过那儿,我并没有在那个地方逗留过很久,但这座城市和莫斯科完全不一样

耶鲁公开课 - 欧洲文明课程节选

I held off for a very long time, but I just got mine a month ago. And now I don't know how I live without it.

我拖了好长时间，一个月前才买的。而且现在我不知道离开它该如何生活。

没有智能手机活不了 - SpeakingMax英语口语达人