G12
When … ? / How long … ? / For and Since

Compare the use of When + past simple and How long + present perfect:

• When did it start to rain?
• It started raining an hour ago / at one o'clock.

• How long has it been raining?
• It's been raining for an hour / since one o'clock.

• When did Joe and Carol first meet?
• They first met a long time ago / when they were at school.

• How long have Joe and Carol 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 (two hours, six weeks, etc.)

• I've been waiting for two hours.
• Sally's been working here for six months.
• I haven't seen Tom for three days.

We use since when we say the start of a period (8 o'clock, Monday, 1985, etc.)

• Sally's been working here since April.
• I haven't seen Tom since Monday.

It is possible to leave out for, but not in negative sentences.

• They've been married (for) ten years. (with or without for)
• They haven't had a holiday for ten years. (with for – negative)

We do not use for + all day / all year / 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)
• 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?)