How much time taken for beginners to learn Singing

A very frequently asked question by beginners , is how much time or how many hours do I have to spend before I can learn Singing ?. 
A simple answer to this question is it takes time to really find the right singing voice and to develop your voice to sing well. Now the question is how much time ?
Here's the thing. Some people have great natural voices, which sounds good. other ones not so great. So natural proficiency and inherent voice quality is certainly helpful.  That's why in my first meeting with new singers I assess their skill grade right at the start. Depending upon what you already have the time take to become proficient will vary. 
When I ask my students what do you mean when you say you want to learn singing. I get different answers from everyone.
  1. Some believe discovering to sing means being able to sing on throw and stay in melody. 
  2. For other ones it means being able to sing high notes well.
  3. For some it means being able to have enough breath command so you don't run out of breath in the incorrect place in a song. 
  4. For some  people want to conquer shyness and be adept to get up and sing without your voice trembling and hands and knees agitating. 
For me as a singing coach and performer it is all of above and even more. 
Mastering singing is about
  1. Breath control 
  2. Singing in correct pitch,
  3. Developing power in your voice range
  4. Seamlessly bridge between your high and low range with smooth transitions 
  5. phrasing,
  6. timing 
  7. Tempo and Rhythm 
  8. Expersing yourself as required by song.
  9. Overall Presentation Style
  10. Stage presence and managing your shyness 
Each of the above will have to be rated on a scale of 1 to 10 and if you are above average in all you can be considered proficient. 
So it depends as a beginner what is your natural proficiency and current proficiency level, and then to what level you desire to learn to sing. If you have a reduced natural proficiency and present skill grade at state 4 out of 10 and you are aspiring for a high grade of awesome 10 out of 10 here's what I'd suggest you to hire a singing teacher who can guide you and quickly get you to the desired level
Not everyone can achieve 10 / 10. So whatever your current level of proficiency, try and keep getting better. Few things you can do to get better.
1) Practice daily, the more hours you spend on regular basis the better you will get. If you spend 1 hour per week then you will not get anywhere. For every 5 to 10 hours spent you should see an improvement in the quality of your output. Of-course I mean 10 hours on one song
2) Keep smoothing your voice by vocal exercises, this will give you control over your pitch and you will develop ability to do riff, runs ,effects ,  belting etc. 
3) Record your voice and listen, Get feedback from trusted sources.