how long和how often的区别在于中文含义不同、用法不同。how long意为多长时间”，主要用来对for加一段时间和长度提问；how often意为每隔多久”，主要用来对频率副词或者状语提问。
how long意为多长时间，主要用来对一段时间(如three days, four weeks 等)提问的。
例句：How long ago was it?
这是多久前的事了?
How long have you been employed at this job?
你受雇做这个工作有多久了？
How long have you lived around here?
你住在这一带有多长时间了？
You sneak a look at your watch to see how long you've got to wait.
你偷偷地望一眼手表，看看要等多长时间。
How long he'll be able to do so is a moot point.
不知道他还能这样做多久。
How long will it take you to knit up this pattern?
你织完这个花样要多长时间？
how often意为每隔多久，主要用来对频率副词或状语(如once a week等)提问的。
例句：How often does he come here? —once a month.
他(每隔)多久来一次?每月一次。
How often do you go to the cinema?—once a week.
你多长时间去看一次电影？—一周一次。
How often do you actually get a gift you really need?
你会多久收到一个你确实需要的礼物？
How often do you water these trees?
你多久给这些树浇一次水？
