How soon通常在一般將來時,意為"再等多長時間?","多長時間才……?"
是對 "in +時間段"（ in ten minutes/two hours/ a week）提問.
Can you tell me how soon you can be ready?你能告訴我你多久能準備好嗎?
how long與how soon都可以用來提問時間.
但是how long一般用于提問完成時,例如：
How long have you been there?-I have been there for 3 years.
而how soon是用于提問將來時的,例如：
How soon will you arrive here?-I will arrive here in 3 hours.
換一個說法.如果答句中用的介詞是for前面就要用how long；如果是in,前面要用how soon
how long指的是多長時間,多長的長度
比如:how long have you been collecting
dolls 你收集玩具多長時間了?
how soon指的是多快,比如how soon will you be there?你多快能到那兒?