We often use "for" and "since" when talking about time.

for + period: a "period" is a duration of time - Ex : for five minutes, for two weeks etc. For means "from the beginning of the period to the end of the period". For can be used with all tenses. Example :
1. They study for two hours every day.
2. They are studying for three hours today.
3. He has lived in Bangkok for a long time.

since + point: a "point" is a precise moment in time - Ex : since 9 o'clock, Since 1st January etc. Since means "from a point in the past until now". Since is normally used with perfect tenses. Example :
1. He has been here since 9am.
2. I had lived in New York since my childhood.