How long does it take to develop female voice?

[HelloWorldVirus]

I want to be an impressionist. I want to learn how to sound like a woman when I speak - I want to annoy my classmates >
Also, can you "yell" or "shout" with your voice without sounding like a child?

[OrderOfOriah]

it all depends on you.  find a quiet place where nobody can hear you and practice practice practice.  I do that with my voice all the time and I am always finding new limits

[HelloWorldVirus]

How do I achieve a natural sounding falsetto like Marcelito (the guy holding shades)? Or are you born with it?

[JenJen2011]

I want to be an impressionist. I want to learn how to sound like a woman when I speak - I want to annoy my classmates >
Also, can you "yell" or "shout" with your voice without sounding like a child?

It will take as long as you want it to. Go look up Candifla on YouTube. She shows a good technique on how to feminize the voice and she sounds amazing.

Yelling/Shouting seems impossible at first. Once you achieve your female voice and get comfortable with it, it'll be much easier for you to shout, scream, laugh loudly, etc.

[Bird]

Took me 7 months of daily training, but I didn't have any professional help.

[toxicblue]

It took me about a year to get semi consistent, but it takes some people just one afternoon. It's really all about "discovering" the voice and working to maintain that discovery, as it doesn't quite as much depend on "developing" it.

[IvyRenee]

I think that Marcelito has a beautiful tenor voice.  At times, it is nearly indistinguishable from a female singing voice, but I feel that he is clearly a tenor.  Aaron from the Rochester Yellowjackets has the same beautiful tenor that comes out at times (see the opening number from this season's "The Sing Off" when they sang "Somewhere Only We Know".  He soars when his solo comes up!).  As for developing a female voice, it takes time, patience, and practice.  I have to practice in private because I'm not out to anyone, and I am still struggling with my identity as a person, but I think that there's a lot of good advice out there to be had.  Just choose carefully and practice, practice, practice.  Record yourself so you know what you sound like to other people.

[HelloWorldVirus]

His modal voice type is baritone. He said, on an interview, that he uses falsetto for sounding like a female so I doubt that. XD

[Alainaluvsu]

It depends on you. I've never really "practiced" and I never fail a blind test. I have a pretty manly voice too, naturally. Others will find it to be the most difficult part of transitioning and can never get it down.

[Sam(my)I am]

Well as others have said it's dependent on you, some people have an easier time achieving it. There's also the matter of practice you basically get out as much as you put in.
I myself have practiced for a month about 10-15 mins  or more a day and I can get into a decent voice (some words I stumble over and "break" my voice) but it takes time for me to enter into my female voice. Also if you can find someone you trust ask if you could practice on them and see what they think.
Also a really nifty tool is to get a quality mic program (Ventrillo would do) and speak into the mic as you practice and listen to the playback and try to adjust.