Each lesson varies, and each person varies: 

• The video Demonstration portion ranges 2-6 minutes. 
• The Challenge portion varies even more depending on the level of difficulty and the strength of your ear. Sometimes a challenge can take only a few minutes, but as things get harder that time can rise to anywhere from 10-30 minutes on average.

The most important thing is that you take your time, soak up the experience, and don't worry about how long it takes. As your skills build, the process will start to feel more fluid and second nature.