I got this question asked as a message on my youtube channel recently and thought I would share the answer.
Q How long does it take to increase your vocal range by one octave.
A.To increase your vocal range by one octave is dependant on your present range. you should look at the quality of your voice more than the range of notes you can produce. Most songs are composed well within two octaves so if you have this range you should be able to sing most songs easily. vocal exercises carried out using the correct technique will increase your vocal range and this should be done slowly note by note over a period of time so that you don't put excess strain on the vocal chords. To inrease by one octave is a bigh achievment if at all possible and without knowing what your range is at present is difficult to say how long or even if you would be able to increase by this amount.