they have worked in the factory for three years,(对划线部分提问)

问题描述:

they have worked in the factory for three years,(对划线部分提问)
how many have they worked in the factory?
但是答案用的是how many呀?所以我有点想不通,答案是不是错的啊

是对three years提问?用how long似乎好些.