1. We use "for" to indicate period of time - for + period : a "period" is a duration of time - for five minutes, for two weeks
2. ' For ' can be used with all tenses.
example:
They are studying for three hours today.
He has lived in Bangkok for a long time.

1. We use "Since" to indicate period of time - since + point : a "point" is a precise moment in time - since 9 o'clock, since 1st January
2. ' Since' is normally used with perfect tenses:
example:
He has been here since 9 am.
He has been working since he arrived.