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.