Out-of-balance vibrations occur whenever the current and the average value of your stress estimate over the last 5 minutes both exceed the threshold.
This is why their frequency cannot be adjusted.
You can, however, adjust the frequency of random vibrations. To do so:
Open the NOWATCH app.
Tap the NOWATCH icon in the upper rightmost corner.
Select Vibrations.
Use the slider to set the desired number of vibrations per day.
Random vibrations will occur throughout the day, up to the selected amount, but will not occur at night.