It should only take you a few moths if you are really dedicated maybe a little longer for the average person.
But there are quite a few things you need to be sure you are doing before you will notice any gains.
First you need to be sure you are going to the gym between three and four times a week consistently.
You will not get any noticeable gains if you go three times one week, two times another and four times the next week.
This is not school there are no make up sessions.
Secondly you need to be eating correctly.
I am not specifically talking about what you are putting into you body although this is also very important.
I am talking about the amount of times you eat.
You need to be eating 5-7 meals a day every 2-3 hours.
This will keep your body energized and "full" and prevent you metabolism from going into starvation mode.
Thirdly you need to be keeping track of almost everything.
When you go to the gym and on what day.
You also want to keep track of what exercises you are doing and how many rep and sets along with how much weight.
This way the following week you can be sure to check your journal and push yourself harder than you did before.
These are just a few things you need to be doing in order to build muscle fast.
Be consistent, eat correctly, and keep track of everything.
Follow those easy steps and you are well on your way to seeing the gains you want and getting the body you desire.