how often常用来问频率；
how long常用来问长度；
how far常用来问距离；
how much常用来问价钱；
how many / how much常用来问数量
How long have you been in your present job?
你干现在这工作多长时间了？
How long have you been living in this country?
你在这个国家居住多久啦？
How much water do you need?
你要多少水？
