We use for + a period of time (two hours, six weeks etc.):
- I've been waiting for two hours.
for: two hours, 20 minutes, a long time, six months, a week, ages, five days, 50 years, years
- Sally has been working here for six months. (not since six months)
- I haven't seen Tom for three days.
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We use since+ the start of a period (8 o'clock, Monday, 1999 etc.):
- I've been waiting since 8 o'clock.
since: 8 o'clock, April, lunchtime, Monday, 2001, we arrived, 12 May, Christmas, I got up
- Sally has been working here since April. (= from April until now)
- I haven't seen Tom since Monday.
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