It mainly depends on the person using the device. We recommend starting at 10 minutes at one instance and working your way up from there. If your neck is in a bad shape, you may need to use it 2 - 3x per day up to 15 minutes (allow at least an interval of 4 hours in between sessions and a strict maximum of 15 minutes per session) at a time but as your range improves and the pain or discomfort subsides, then you can also decrease the frequency and duration of the stretch. However, should the 15-minute duration give you either a headache, or feeling of dizziness/nausea, or back pain, then you may need to decrease the time and place the pad more towards the middle of your neck.
We like to compare this in improving hamstring flexibility. If you have REALLY tight hamstrings, then you are probably going to need to stretch them 2 - 3x a day for a few weeks to see significant physical change. If you have relatively flexible hamstrings, then you probably only need to stretch them 3 - 4x per week.
There is no exact number as the objective is to feel a comfortable, gradual stretch and much of it depends on how it feels for you. Ideally, we recommend that the base/pad of Turbo NeckSling be approximately 2-4 inches off the ground when assembled, however, as for the distance between the hammock and the y-axis (e.g. wall or door), it will also depend as to which angle feels relaxing and comfortable for you.
If you are seeking a more aggressive stretch, scoot away from the wall or door. Start more conservatively and see how your body responds after 1 session.
If the stretch is too much, move back towards the wall or door.
Your body is very smart, it knows what it likes and doesn't like. If the stretch feels too strong, then it probably is. Adjust the tension based on Your needs.