Since vs. For

For is used when we measure the duration, when we say how long something lasts.

For + a period of time: To measure a period of time up to the present, we use the present perfect tense and not the present tense.

E. g.

I have known her for a long time.

The present tense with 'For' refers to a period of time that extends into the future.

E. g.

How long are you here for?

Since gives the starting point of actions, events or states. It refers to when things began.

Since + a point in time (in the past), until now.

E. g.

I've been waiting since 7 o'clock.

With since we use the present perfect tense or the past perfect tense.

E. g.

I have been here since 5 o'clock and I am getting tired.

Since can also be used in the structure It has been + period of time + since.

E. g.

It has been two months since I last saw her.