— How long have they been studying together ? (they / study together) — They 've been studying together since 2015. — How long ? — It the afternoon. — How long you / have that bag ? — Let me think. I four months. — How long Jack / work here? — He he graduated from the university. — How long Mark and Ellie / be married ? — They 15 years. — How long Tina / learn Spanish ? — She three years.

— How long have they been studying together? (they / study together)
— They've been studying together since 2015.

In this question, we are asking about the duration of time that "they" have been studying together. The answer states that they have been studying together since 2015. The phrase "have been studying" indicates that the action started in the past and is still continuing in the present.

— How long?
— It's been the afternoon.

In this question, we are asking about the duration of time it has been the afternoon. The answer "It's been the afternoon" means that the time period of the afternoon is currently ongoing.

— How long have you had that bag?
— Let me think. I've had it for four months.

In this question, we are asking about the duration of time the speaker has had the bag. The answer "I've had it for four months" means that the speaker has owned the bag for a period of four months.

— How long has Jack worked here?
— He has worked here since he graduated from the university.

In this question, we want to know the duration of time Jack has been working at the current place. The answer "He has worked here since he graduated from the university" implies that Jack has been working here since his graduation.

— How long have Mark and Ellie been married?
— They have been married for 15 years.

In this question, we are asking about the duration of Mark and Ellie's marriage. The answer "They have been married for 15 years" indicates that Mark and Ellie have been married for a period of 15 years.

— How long has Tina been learning Spanish?
— She has been learning Spanish for three years.

In this question, we want to know the duration of time Tina has been learning Spanish. The answer indicates that Tina has been learning Spanish for a period of three years.