"मैं आ रही हूँ।"
Translation:I am coming.
Is it possible to simply answer 'आ' if someone is asking me how long he/she will have to wait (just in order to give a very quick response like in English)?
Simply saying "आ" would be like saying "Come!", as in you are asking the other person to come. (Note: It is in the first level of formality. More formal forms are आओ and आइये).
"Coming!" would be "आ रही हूँ" (i.e, omit the मैं).