How Long have you

Study this example situation:

Dan and jenny are married. They got married exactly 20 years ago, so today is their 20th wedding anniversary.

They have been married for 20 years.
We say: They are married. (present)

but  How long have they been married? (present perfect)
(not How long are they married?)
They have been married for 20 years.
(not They are married for 20 years)

We use the present perfect to talk about something that began in the past and still continues now.

• Paul is in hospital.
  but  He's been in hospital since Monday. (= He has been ... )
  (not Paul is in hospital since Monday)
• Do you know each other well?
  but  Have you known each other for a long time?
  (not Do you know)
• She's waiting for somebody.
  but  She's been waiting all morning.
• Do they have a car?
  but  How long have they had their car?

I have known/had/lived etc. is the present perfect simple.
I have been learning / been waiting / been doing etc. is the present perfect continuous.

• I've been learning English for six months.
• It's been raining since lunchtime.
• Richard has been doing the same job for 20 years.
• 'How long have you been driving?'  'Since I was 17.'

• How long have you known Jane? (not have you been knowing)
• I've had a pain in my stomach all day. (not I've been having)

• Julia has been living / has lived here for a long time.
• How long have you been working / have you worked here?

• I've always lived in the country. (not always been living)