The prepositions for and since in English are used to indicate the duration of something that is happening.
For is used in combination with a period of time (2 hours, 6 weeks, etc.)
For + a period of time (two hours, six weeks etc.):
- I've been waiting for two hours;
- Sally has been working here for six months, (not since six months);
- I haven't seen Tom for three days. (not since three days).
Since is used in combination with the start of a period of time (8 o'clock, Monday, 2010, etc.):
Since + the start of a period (8 o'clock. Monday. 1999 etc.):
- I've been waiting since 8 o'clock;
- Sally has been working here since April;
- I haven't seen Tom since Monday.
Omitting for:
It is possible for for to be omitted (most often, but not always – 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).
For is not used in combination with all – for + all (all day / all my life etc.):
- I've lived here all my life, (not for all my life).
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