How soon通常在一般将来时,意为"再等多长时间?","多长时间才……?"
是对 "in +时间段"（ in ten minutes/two hours/ a week）提问.
Can you tell me how soon you can be ready?你能告诉我你多久能准备好吗?
how long与how soon都可以用来提问时间.
但是how long一般用于提问完成时,例如：
How long have you been there?-I have been there for 3 years.
而how soon是用于提问将来时的,例如：
How soon will you arrive here?-I will arrive here in 3 hours.
换一个说法.如果答句中用的介词是for前面就要用how long；如果是in,前面要用how soon
how long指的是多长时间,多长的长度
比如:how long have you been collecting
dolls 你收集玩具多长时间了?
how soon指的是多快,比如how soon will you be there?你多快能到那儿?