When and How Long indicate different things. When usually indicates a specific point in time, or something that is considered as a specific point. How long indicates a period or length of time, with a beginning and ending point.

When did you move to Arizona? I moved here in 1997. How long have you lived in Arizona? I have lived here since 1997.

(Asks for specific time) (Asks for length of time.)

Notice that with when and how long, different tenses and different verbs are often used.

When did you buy that car? I bought it two months ago. How long have you owned that car? I have owned it for two months.

(simple past tense) ("buy” indicates action) (present perfect tense) ("own” indicates possession)

“Actions” usually happen at a point in time, whereas things such as “possession,” “status” “condition” “awareness” refer to something that continues over a period of time. (See: Grammar: Action vs. Status.)

For example,

action (When did you….?) meet your best friend get that new watch become a lawyer

status/condition (How long have you …?) known your best friend had that new watch been a lawyer

A common mistake is using an “action” verb to indicate something that exists over a period of time.

How long have you bought that car? How long have you had that car?

Incorrect! Correct

In the above statement, the present perfect tense is used to indicate that you still have the car now. How long can also be used to indicate conditions that existed totally in the past.

How long did you live in Denver? I lived in Denver for two years.

(You do not live there now.) (from 1997 to 1999)

Compare the following:

A. Wholly in the past How long were you in Florida. I arrived there in May. I left there in July. I was there for two months.

B. Ongoing at the present How long have you been in Florida? I arrived here in May. I am still here. I have been here since May.