Compare When…? (+ Past Simple) and How long…? (+ Present Perfect):
A: When did it start raining?
B: It started raining an hour ago / at 1 o’clock.
A: How long has it been raining?
B: It’s been raining for an hour / since 1 o’clock.
A: When did Joe and Carol first meet?
B: They first met a long time ago / when they were at school.
A: How long have Joe and Carol known each other?
B: 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 (two hours, six weeks etc.):
* I’ve been waiting for two hours.
* Sally’s been working here for six months. (not since six months)
* I haven’t seen Tom for three days. (not since three days)
We use Since when we say the start of a period (8 o’clock, Monday, 1985 etc.):
* I’ve been waiting since 8 o’clock.
* Sally’s been working here since April. ( = from April until now)
* I haven’t seen Tom since Monday. ( = from Monday until now)
It is possible to leave out For (but not usually in negative sentences):
* They’ve been married (for) ten years. (with or without For)
* They haven’t had a holiday for ten years. (you must use For)
We do not use For + all… (all day / all my life etc.):
* I’ve lived here all my life. (not For all my life)
We say “It’s (a long time / two years etc.) Since something happened”:
* It’s two years since I last saw Joe. ( = I haven’t seen Joe for two years / the last time I saw Joe was two years ago)
* It’s ages since we went to the cinema. ( = We haven’t been to the cinema for ages)
The question is How long is it since…?
* How long is it since you last saw Joe? ( = When did you last see Joe?)
* How long is it since Mrs Hill died? ( = When did Mrs Hill die?)