One of the main reasons that people fail to build muscle mass is because they do not understand why, how, and when muscles increase in size.
Most people believe that lifting the weights is what causes the muscles to get bigger and that the more often they can lift weights, the quicker their muscles will get bigger.
But that's wrong.
Their error is in stopping with the first step of building muscle mass.
The first step is to lift the weights, but that is ONLY the first step.
The idea is to lift weights and really stress the muscles that you want to make larger.
The muscles need to be stressed to the point that small tears occur.
The muscle is actually damaged.
The second step, and the step that actually causes the muscle to get bigger, is rest.
The body is magnificent.
It is designed to heal itself.
When you have lifted weights and created enough stress to cause them to develop small tears, when you rest, your body goes to work repairing the damage.
The body repairs the tears in the muscle by building more tissue - more MUSCLE tissue.
As the muscle heals, it gets bigger because there is more muscle tissue.
Working out longer isn't going to make your muscles get bigger faster.
The body repairs the damage so that the next time the muscle is required to lift that much weight, it will be strong enough to lift it without causing any damage.
So your answer to that is to ask the muscles to lift heavier weights the next time.
You have to put more stress on the muscles, but doing it longer doesn't make it better.
When the muscles have been stressed and the small tears have occurred, you've done all you can do.
The rest is up to your body's repair system.