We often use for and since when talking about time.

A period is a duration of time, for example: 5 minutes, 2 weeks, 6 years. For means "from the beginning of the period until the end of the period." For can be used with all tenses.

since + point

A point is a precise moment in time, for example: 9 o'clock, 1st January, Monday. Since means "from a point in the past until now." Since is normally used with perfect tenses.

For can be used with all tenses. Here are a few examples:

• They study for two hours every day.
• They are studying for three hours today.
• He has lived in Bangkok for a long time.
• He has been living in Paris for three months.
• I worked at that bank for five years.
• Will the universe continue for ever?

For is NOT used with "all day", "all the time" etc.

• I was there all day. (not *for all day)

Since is normally used with perfect tenses:

• He has been here since 9am.
• He has been working since he arrived.
• I had lived in New York since my childhood.

Since can also be used in the structure "It is [period] since...":

• It is a year since I saw her.
• How long is it since you got married?