Ihave been talking for five minutes.

hace cinco minutos que yo

hace cinco minutos que yo "estoy hablando"

Explanation:

Even if the answer you gave on the test, which was: "hace cinco minutos que yo hablo", is technically correct, the sentence is awkward and essentially never used by native spanish speakers. On the other hand, "estoy hablando" would be the words use by a native most of the time, especially if they are from Latin America. In this case, "estoy hablando" would be equivalent to "I have been talking".

In Spain, however, the most common form would be: "hace cinco minutos que he estado hablando", "he estado hablando" being the same (literally the same translation, word by word) as "I have been talking".