简单说来就是 "How long"+"动"+"宾"
"How long"有两种意思：
多长-
How long is the Great Wall of China?
中国的长城有多长.
多久（多长时间）-
How long have you been sick?
你病了多长时间了?
How long were you hospitalized for?
你住院以来多长时间了?
用法跟陈述句有关.
如果是说中国的长城有多长,陈述句是：
The Great Wall of China is xxxx kilometers long.
在"How long"的后面加上be动词,再把剩下的句子加上,就是：
How long + "be" + "object" =
How long is the Great Wall of China.
如果是指时间,陈述句是：
You have been sick for a week.
在"How long"的后面加上动词（或助动词/"be"动词）,再加上要问的.如果有助动词,那么助动词在前面,然后是主语,然后是动词+剩下的句子：
How long + "be" + "object" =
How long "have"（助动词）+ "you"（主语）+ "been"（be动词）+ sick?
How long have you been sick?
其实用法同其他问句或倒装句.
希望回答了你的求助.