楼上的解释的很好 我再补充一点
how soon 通常是对in+一段时间的提问
如 He will be back in five minutes. 如果把in five minutes 划上,就它来提问,就应该是楼上的那句:How soon will he be back?
How long通常是就时间长短来提问的,如:It takes me two hours to get to school. 如果把two hours 划上,就它来提问,就应该是:How long does it take you to get to school?
另外补充一个how far,它是就距离来提问的,如:The school is two kilometers away from my home. 按要求变为特殊疑问句就应该是:How far is it from the school to your home?
以上 希望有所帮助 ^_^