HOW LONG ....? + with the present perfect

refer :
http://www.englishteachermelanie.com/grammar-present-perfect-how-long-have-you/

You can use the present perfect tense when you want to talk about how longyou have done something, or for what amount of time you have done something. It is used to talk about an action that began in the past and continues up to the present (and will probably continue in the future). This is the important thing to remember about using the present perfect in this situation: the activity, action, or event is still going on. It has not finished.

I have lived in Paris for 7 years.

I have been a vegetarian since I was a young boy.

I have worked for this company for 4 months.

I have loved you since the day I met you!

We have been married for 30 years.

Remember: The simple past is used to talk about an action that started and FINISHED in the past. The present perfect is used to talk about an action that started in the past and CONTINUES in the present.

Compare:

I have studied French for 10 years. (present perfect)

(I started studying French 10 years ago, and I am still studying it now.)

I studied French for 10 years. (simple past)

(I started studying French 10 years ago. I finished studying French 2 years ago. I am not studying French now.)

Did you notice that I used for 10 years in the above example. Why did I use for? Could I have used since in the above examples?

for + a period of time:
for 6 years, for 5 months, for 4 days, for a long time

since + a point in time in the past:
since 2008, since I was a child, since last month, since yesterday.

I could have used since, but only with the present perfect:

I have studied French since 2000.

X:
I studied French since 2000.

Did you notice that none of these sentences include ago? Ago is not used with the present perfect. It is used with the simple past.

Compare:
Do you know Catherine?
~ Yes I know Catherine.
(simple present – I know her now.)

How long have you known her?
~ I’ve known her for 6 six years / since 2004.
(present perfect)
~ I met her 6 years ago. We were in the same class at school.
(simple past)

~

More example sentences using the present perfect:

My mom is in New York City on a business trip. She has been there since Monday.

I live in Canada. I have lived in Canada my entire life.

Jen and Mark have been married for 25 years.

It has been such a miserable day. It has been raining all day!

Materials from :
http://www.englishteachermelanie.com/grammar-present-perfect-how-long-have-you/