We often use for and since when talking about time.
A. for + period: a "period" is a duration of time - five minutes, two weeks, six years. For means "from the beginning of the period to the end of the period".
Example :
1. They study for two hours every day.
2. They are studying for three hours today.

B. since + point: a "point" is a precise moment in time - 9 o'clock, 1st January, Monday. Since means "from a point in the past until now".
Example :
1. He has been here since 9 am.
2. He has been working since he arrived.