Typically "It has been a long time since (someone) has (done something)" implies they haven't done something in a long time. Do you mean you haven't studied English for a long time? In that case it should be in the present perfect, not present perfect continuous (It has been a long time since I have studied English). The past simple also works (It has been a long time since I studied English). Example: It has been a long time since I have traveled abroad = I haven't traveled abroad in a long time. You can use 'since' to express your idea, but as you are 'since' to give duration (say how long) and not to say give a start time, you should use 'ago'. I have been studying English since a long time ago. Alternatively, you can just use 'for'. I have been studying English for a long time.

No, here you'd just use the past simple because you're expressing a point in the past since something else has been happening. Look at similar examples: I've been studying English since 2017. I've been studying English since my visit to the USA in 2017. I've been studying English since I went to the USA in 2017. Hope it makes sense to you :)

Hello, You could say "It's been so long since I started to study English"

I’ve been studying English for a long time. (Correct and clear) It’s been a long time since I’ve been studying English. (Incorrect. It’s just a longer and inferior way to say the first statement, though it would be understood.) It’s been a long time since I started to study English. (Correct with a similar meaning to the first statement. It has a slightly difference nuance though - it makes you think more about the past and the start. You could also use ‘studying’ instead of ‘to study’ here.)

I think if you want to use since for this kind of statement you should specify the exact or at least estimate the time at which you started studying the language in question in order to avoid the redundancy of using 'been' twice. So for example, you'd say: "I have been studying English since 2014" OR "I have studied English since 2014" .