Long enough to turn white on the surface instead of melting. We get light flurries that just melt on contact with the street. When the temps have been under 20 for a few days, the snow lands on the street like dry sand and blows around. Subzero temps grow hoar frost on the trees. I don't mind seeing a couple feet out the front door. Takes about 20 minutes to shovel the driveway and steps.
Hoar Frost