How often ...

• （问时间频率）多常时间（一次），多久（一次）

How often ...的用法和样例：

例句

1. How often do you hear from your sister?
   你多长时间接到你姐姐一次信?
2. How often do you have your car greased?
   你多久才给小轿车上一次油？
3. How often do you go to the library?
   你多长时间去图书馆一次？
4. How often do you wash your hair?
   你多长时间洗一次头？