1、表示多長時間，主要用來對一段時間(如three days, four weeks 等)提問,答語通常是（for）three days/weeks/months等時間段,它可用于各種時態(tài)。
It did not take us long to nail that lie.
沒多久我們就把謊言揭穿了。
He is sure to spill the beans before long.
用不了多久他準會走漏消息的。
How long does it take to get to London from here? 從這里到倫敦要多長時間?
At least ten hours. 至少要10個小時。
2、表示某東西有多長。如：
How long is the river? 這條河有多長？
About 500 km. 大約500千米。
How long is the tunnel? 隧道有多長？
You sneak a look at your watch to see how long you've got to wait.