Since / For
Since -- indicates a point in time.(since + starting point)
e.g - I have lived in Bangalore since 1991.
e.g - It's been raining since the beginning of the month.
For -- indicates the length of a period of time.(for + period of time)
e.g - I have lived in Bangalore for ten years
e.g - My mother will be in Delhi for the next ten days.
She'd been working there since a long time - Incorrect
She'd been working there for a long time - Correct