Present perfect progressive or future: "have been saving up" or "going to save up"?

Question

Put the verbs in brackets into a suitable tense form

I ______ (save up) for three years because I want to buy a guitar.

Although there is a period mentioned, yet I think using the present perfect progressive is not much suitable

It seems to me it is a plan or a decision someone decided to in advance.

I have been saving up (save up) for three years because I want to buy a guitar.

My answer or my suggestion:

I am going to save up (save up) for three years because I want to buy a guitar.

Can I use ‘will’: I will save up (save up) for three years because I want to buy a guitar.

Answer 1

Both of your choices are grammatical, but knowing in advance exactly how long it will take you to save up (I am going to save up for three years or I will save up for three years) is the less likely choice, but that is only because life is unpredictable to a degree. The price of the guitar might change. You might get a raise. You might change your mind and decide the trombone is better. It is more likely that you know how long you have been saving up; it has been an ongoing plan:

I have been saving up for three years because I want to buy a guitar.

But a future plan of action is certainly possible.

I am going to save up for a guitar. It will probably take me about three years.