It depends on what you are trying to ask. "How long have you been studying English?" is a question that many people will consider to mean "How long has it been since you began studying English?" This form of the question will make people measure from the day they began to the present. For example, someone might answer, "I have been studying English for two years." The other option---"How long are you studying English?"---is in the present progressive, so it's like asking "How long are you going to continue studying English tonight?" This form of the question will make people measure into the future and tell you when they are going to stop studying. For example, someone might answer, "I will study English until 9 p.m." or " I am going to study for another two hours."

Hi Júnior, "How long have you been studying English?" is the correct way to ask about someone's history of studying the language up through today. However, you can ask: "How long are you studying English for?" if you want to know how long the person plans to continue studying English. I hope I've helped answer your question. Please feel free to ask if you have any more.

How long have you been studying English for? O de manera más coloquial quitándole el "for" al final (tal como la opción número uno en tu pregunta)... espero te ayude esta respuesta :)