看原句的语境和意思了
How long对时间的提问一般情况下用完成时,表示“……多长时间了”,
eg.I have been looking for it for 3 days.
How long have you been looking for it
但也有：
eg.I used to look for it for 3 days.
How long did you look for it
看出来区别了吗?主要是原句.学英语,语境和语感很重要,这和你的阅读量有关,我虽学的不是很好,但感触颇深,总之,自信点,