Compare When…? (+ past simple) and How long…? (+ present perfect):
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.
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.
- Sarah’s been working here for eight months. (not “since eight months”)
- Carol’s been working here since December. (= from December until now)
- I haven’t seen Mark for four days. (not “since four days”)
- I haven’t seen George since Wednesday. (= from Wednesday until now)
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 haven’t had a holiday for three 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 Kevin. (= I haven’t seen Kevin for two years / the last time I saw Kevin 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 Betty? (= When did you last see Betty?)
- How long is it since Smith died? (= When did Mr. Smith die?)