We often use for and since when talking about time.
for + period
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.
