Well, any significantly difficult skill is going to take months to get no matter what. At first you may not be able to work the skill that much. With the back lever, one would typically be limited by pangs of tendonitis in the elbows. As you get stronger and more familiar with the skill you can work it more. Half of it is your muscles, joints and tendons developing conditioning and the other half is your CNS getting more efficient at the movement pattern.