[dendrite9]

It is highly dependent on the snow conditions and the recent weather. Sometimes even just the a couple hours are enough to change the conditions to have a good chance of rollerballs. The climate also has an impact, in my experience more coastal areas have more periods when they form.

And in some cases the rollerballs get too tall for the bonding strength of the snow, so they break into parts that can restart the cycle if the slope is steep enough.