Use of For:
For is going to be used to describe the period.(Duration of time)
Use of Since:
Since in going to be used to describe specific point in time.
Some people think that they are not supposed to use since when a number is concerned. This is not true. You can use since for a number, but it should be in specific point in time. It should not be used to describe the duration of time.
I have not eaten since 4 a.m. (Specific point in time)
I have not eaten for the last 4 hours (Duration)