1、表示多长时间，主要用来对一段时间(如three days, four weeks 等)提问,答语通常是（for）three days/weeks/months等时间段,它可用于各种时态。
It did not take us long to nail that lie.
没多久我们就把谎言揭穿了。
He is sure to spill the beans before long.
用不了多久他准会走漏消息的。
How long does it take to get to London from here? 从这里到伦敦要多长时间?
At least ten hours. 至少要10个小时。
2、表示某东西有多长。如：
How long is the river? 这条河有多长？
About 500 km. 大约500千米。
How long is the tunnel? 隧道有多长？
You sneak a look at your watch to see how long you've got to wait.