Using "Since" and "For" in Perfect Continuous Tenses:
The Use of "Since" in English Sentences:
Since is used in sentences where the beginning or start time of an action is mentioned and it is difficult to determine for how long the said action has has been in progress.
Examples may include:
a) He had been going to school since August.
The repeated action of "going to school" took place in the past and has been continuing till reporting the of the matter.
b) I had been doing my work since 6 O' Clock.
Similarly, the "doing" began at 6 and still continues either continuously or continually.
The Use of "For" in Perfect Continuous Tense:
For is used in sentences where action is in progress for time which is already counted e.g.
a) He had been going to school for 6 months.
b) I had been doing my work for 5 days.
Note: All continuous (progressive) tenses would take present participle form of verb.