I do not know how long they lasted.

我不知道这种声音持续了多久。

youdao

I do not know how long, now this is my job.

我不知道能做多久，现在这是我的工作。

youdao

I will not help, I think of as Piaomiao do not know how long.

我的无助，我的思随不知道要缥缈多久。

youdao

We are worried because we do not know how long they will stay or how we will take care of them.

VOA: standard.2009.10.27

People do not know how long they will live, and so they keep more money than they turn out to need.

ECONOMIST: Death and taxes

Mr. THOMPSON: I don't know how long Mr. Rumsfeld intends to stay but I do know this - he's not going to leave under fire.

NPR: Donald Rumsfeld, Secretary of Self-Defense