how soon指再过多久， 主要对表示将来的时间进行提问，比如：in an hour, in three weeks。how long表示多长时间，与连续性动词连用，主要用于对一段时间进行提问，常用for+时间段回答。how often主要强调动作发生的频率。

How long

How long多久了（问时长）,以For+时间段或since +时间点作答

How long did you stay here? I stayed here for two weeks.

How soon

How soon多久之后（问解决事情等的速度）,用in+时间段作答

How soon will your father be back? In two weeks.

How often

How often多经常（问频率） ,常以数词+ times a week（month 、year……）作答

（强调做了几次）如果句子中有always, usually, often, never, once a day, twice a week, three times a year等词时，常用how often 提问。

How often does your brother go home? Twice a year.