That is an excellent question and there is no definitive answer because it depends on you, the individual. I have been told that it takes 3 weeks to make something a habit. That may be true for some things but I am hesitant to tell anyone that his/her 'real' voice will be a habit in 3 weeks.
I did have a young woman who made it a habit in less than 3 weeks and I attribute her success to the fact that she was married, did not have a job and had no children. I believe that she worked on her voice day in and day out. Most of us, however, are not in that situation.
Another man I worked with made it a habit very quickly because he was a taxi dispatcher. 600 times a day he would answer the phone in his 'real' voice. Trust me, 600 times a day makes it a habit pretty darned quick!
What is more likely is what I just heard from Shaina. Shaina started her training in the beginning of July, had a few phone conversations with me during August, and sent me an email on the 31st of August saying,
"I feel as though I've finally made my optimum pitch a habit, which makes me feel so great and I actually love listening to my new voice versus old!!! Thank you again! You've helped me in so many ways- personally and professionally!"
Does that mean you can make it a habit in 6 or 7 weeks? Possibly. But you must consider how motivated and dedicated you are. Learning the techniques involved to find your 'real' voice is not difficult. What takes effort is your awareness. During voice training, you will be in the process of retraining your inner ear to recognize your 'old' voice versus your 'real' one, as well as remembering to use your new one as much as possible. That is your homework. The more you use it, the sooner it will become a habit.
Whether it takes you 3 weeks, 3 months, or even longer, the benefits of discovering your richer, warmer, more mature-sounding voice are worth the effort. Just ask Shaina.