Your NuCalm session is as long as you want it to be or need it to be. Ideally, you want to go to your natural endpoint - when you just "wake up" on your own. If time is limited, you can set a timer on a separate device with a minimum length of 20 minutes.
It is normal for initial session lengths to be quite long as your body is catching up from years of imbalance, stress and poor sleep. Ultimately, your natural endpoint time will begin to decrease as NuCalm becomes a tool for daily maintenance. The length of your natural endpoint typically correlates to your current stress level (high stress, long NuCalm journey; low stress, short NuCalm journey). Sleep deprivation, poor sleep quality, stress, jet lag, illness, injury are all factors that will affect the length of your session.