In my experience, it's usually 24-48 hours after the snowfall ends so there's plenty of time to get it done. Also in my experience, it is very rarely enforced. In many places, the sidewalk is the only safe place to walk the street so it's a real drag when someone doesn't clear their sidewalk. You either trudge through the snow or risk life and limb walking in the street. If you own property, you're responsible for keeping the sidewalk clear and it's your duty to the community to do it.