Compare When…? (+ past simple) and How long…? (+ present perfect):

When did it start raining? It started raining an hour ago. How long has it been raining? It’s been raining since 3 o’clock. - English Grammar - When...? and How long...? For and since

“How long has it been raining?” “It’s been raining for an hour (since 3 o’clock).” - English Grammar - When...? and How long...? For and since
“How long has it been raining?” “It’s been raining for an hour.”

 

When did John and Lisa first meet? They first met when they were at school. - English Grammar - When...? and How long...? For and since

How long have John and Lisa known each other? They’ve known each other for a long time / since they were at school. - English Grammar - When...? and How long...? For and since
How long have John and Lisa known each other? They’ve known each other for a long time / since they were at school.

 

We use both for and since to say how long something has been happening.

We use for when we say a period of time (three hours, seven weeks etc.):

  • I’ve been waiting for three hours.

I’ve been waiting for three hours. - English Grammar - When...? and How long...? For and sincefor three hours (a week, 30 minutes, 50 years, four days, a long time, six months, ages) - English Grammar - When...? and How long...? For and since

We use since when we say the start of a period (9 o’clock, Tuesday, 1988 etc.):

  • I’ve been waiting since 9 o’clock.

I’ve been waiting since 9 o’clock. - English Grammar - When...? and How long...? For and sinceSince 9 o’clock (Wednesday, Christmas, February 8, lunchtime, December, they were at school) - English Grammar - When...? and How long...? For and since

  • Sarah’s been working here for eight months. (not “since eight months”)
Sarah’s been working here for eight months. - English Grammar - When...? and How long...? For and since
Sarah’s been working here for eight months.

 

  • Carol’s been working here since December. (= from December until now)
Carol’s been working here since December. - English Grammar - When...? and How long...? For and since
Carol’s been working here since December.

 

  • I haven’t seen Mark for four days. (not “since four days”)
I haven’t seen Mark for four days. - English Grammar - When...? and How long...? For and since
I haven’t seen Mark for four days.

 

  • I haven’t seen George since Wednesday. (= from Wednesday until now)
I haven’t seen George since Wednesday. - English Grammar - When...? and How long...? For and since
I haven’t seen George since Wednesday.

 

It is possible to leave out for (but not usually in negative sentences):

  • They’ve been married (for) five years. (with or without for)
They’ve been married (for) five years. - English Grammar - When...? and How long...? For and since
They’ve been married (for) five years.

 

  • They haven’t had a holiday for three years. (you must use for)
They haven’t had a holiday for three years. - English Grammar - When...? and How long...? For and since
They haven’t had a holiday for three years.

 

We do not use for + all (all day / all my life etc.):

  • I’ve lived here all my life. (not “for all my life”)
I’ve lived here all my life. - English Grammar - When... and How long... For and since
I’ve lived here all my life.

 

We say “It’s (a long time / two years etc.) since something happened”:

  • It’s two years since I last saw Kevin. (= I haven’t seen Kevin for two years / the last time I saw Kevin was two years ago)
It’s two years since I last saw Kevin. - English Grammar - When...? and How long...? For and since
It’s two years since I last saw Kevin.

 

  • It’s ages since we went to the cinema. (= We haven’t been to the cinema for ages)
It’s ages since we went to the cinema. - English Grammar - When...? and How long...? For and since
It’s ages since we went to the cinema.

 

The question is How long is it since…?

  • How long is it since you last saw Betty? (= When did you last see Betty?)
How long is it since you last saw Betty? - English Grammar - When...? and How long...? For and since
How long is it since you last saw Betty?

 

  • How long is it since Smith died? (= When did Mr. Smith die?)
How long is it since Mr. Smith died? - English Grammar - When...? and How long...? For and since
How long is it since Smith died?