—TV do you watch every day?--About two hours.

how long,how soon和how often都含有how,并且都和“多长时间、多久”有关,但它们的用法不相同.
how long意为“多久、多长时间”,主要是对一段时间进行提问,答语通常是（for）three days/weeks/months等时间段,它可用于各种时态.
How long do you stay in Beijing every year?每年你在北京住多久?
How long have they lived here?他们住在这里有多久了?
—How long were you not at school last year?去年你多久没上学?
—About two weeks.约两周.
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how soon意为“还要多久”,是对从某个基本时间到将来某动作结束或某动作发生这段时间提问,常用在一般将来时态的句子中,其答语通常是“in + 一段时间”.
—How soon can you finish the work?还要多久你能完成这项工作?
—In half an hour.半小时后.
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how often意为“多久……次、是否经常”,用来提问在某一特定的时间进行某个动作的次数,答语通常是always,usually,often,sometimes,once/twice a day/month等.
—How often do you get to school very early?你多久早到校一次?
—Usually.通常.
—How often do you see a film?你多久看一次电影?
—Once a month.每月一次.

这道题填How long，因为后面回答的是时间，而不是频率。
how often 提问频率，how soon提问多快，how long提问多久，后面两个有时可以通用答案上写的是how much,为什么？- -我看成你问 watch TV了，不好意思。 是how much，它间接问 看电视的时间，how much TV=how long watch TV...

How often 询问“频率”，如：How often do you come here on foot? Every day.
How soon 常正对“in+一段时间（……以后）”提问，如：How soon will you come back? In a week.
How long 有两个作用。1）询问时间长短，如：How long may I keep the boo...