That depends on your body type-some will gain mass faster than others.

But don’t use genetics as an excuse. No matter what your genetic makeup, HOW you train-combined with the right diet-is BY FAR the most important factor in the results you get.

So why do 95% fail to gain mass?

Because they train the wrong way. Most people unknowingly take the path of MOST RESISTANCE-meaning they waste a lot of time on workouts that don’t get them closer to their goal.

When your goal is building muscle, you only want to focus on the activities that will get you there the fastest-anything more is counterproductive.

In other words-DISCARD any and every exercise that will not help you gain muscle mass the fastest.

The best way to do that is to ignore “conventional wisdom”. Doing what everybody else does is a surefire way to get the results everybody else gets. And since 95% of gym-goers fail to gain mass, this isn’t the smartest option.

What’s the SINGLE most important factor in gaining mass?

Time under load (TUL)-this is the ONLY measurement you should be paying attention to. Quite simply, the higher this number is, the better.

Why?

Because the more time you spend under load, the better the chances of your tissue size expanding and you getting bigger.

Everybody knows this-nothing new here

Maybe so-but many people mistakenly train for least time under tension THINKING they are training for the most.

How?

By engaging in “failure training”. Don’t get me wrong-I know people who’ve seen spectacular results with this method.

But it’s FAR from the best

Assuming you follow the conventional “failure training” guidelines-one set to failure per exercise-your TUL is limited to how long that one set takes you.

However, what if you were to do 10 sets of the same number of reps? You just multiplied your TUL by 10.

Increasing your number of sets is how to get results FAST

So how long does it take to build muscle? When you follow the above advice, you should begin seeing results in14-30 days–if not sooner.

Source: all-bodybuilding
“How Long Does it Take to Build Muscle? Here’s the Truth”