Since vs For

Since / For

Since indicates a point in time.(since + starting point)

• I have lived in Bangalore since 1991.
• It's been raining since the beginning of the month.

For indicates the length of a period of time.(for + period of time)

• I have lived in Bangalore for ten years
• 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