Since and For

 For

For+Period-a "period" is a duration  of time.

For two weeks,for five minutes,for 6 years

• They study for two hours everyday.
• He worked at that bank for 5 years.

Since 

Since+Point-a "point" is a precise moment in time- 9 o'clock,1 st january.

• He has been here since 9 am.
• He has been studying since 7am.