* He estado estudiando: This is the most common and versatile translation. It uses the present perfect tense and implies that the studying has been ongoing and may still be continuing.
* Llevo estudiando...: This translates to "I've been studying..." and is followed by a time period (e.g., "Llevo estudiando dos horas" - I've been studying for two hours). This emphasizes the duration of the action.
The choice depends on what you want to emphasize. If you just want to say you've been studying, "He estado estudiando" is perfect. If you want to emphasize *how long* you've been studying, use "Llevo estudiando...".