For / since

My grandparents walk _____ twenty minutes a day.

• for

• since

How long is it _____ you visited your grandparents?

• for

• since

We have been dating _____ a long time.

• for

• since

That lady has been waiting for the bus _____ noon.

• for

• since

I studied for this test _____ three days straight.

• for

• since

My family has lived here _____ last March.

• for

• since

She has worked there _____ she finished college.

• for

• since

I have been calling you _____ half-an-hour.

• for

• since

Lisa and Jill have been at the library _____ a long time.

• for

• since

My teacher has been sick _____ last Monday.

• for

• since