Both ‘For' and ‘Since' are used in the Perfect Continuous Tense.
Remember the following points regarding the use of ‘for' and ‘since' in this tense:
(a) ‘For' is used to suggest the period of time for which an action has been going on:
Examples: Nidhi has been sleeping for two hours.
Reshma has been living in Delhi for three months.
(b) ‘Since' is used to suggest the point of time at which an action started:
Examples: Nidhi has been sleeping since morning.
Reshma has been living in Delhi since 2006.
We can also say that if a sentence contains any digit like two, three, four or numeral like one, two, three etc. use ‘For' and if it contains words like morning, evening, afternoon, O'clock, last etc. use ‘Since'.
Note: If ‘last' is preceded by ‘the', always use ‘For' before it.
As given above.