Officials in Clark County, Washington have determined that a smoking area should be a minimum of 25 feet away from any doors, open windows or vents. This is the distance that they have determined to safely prevent smoke from entering into the building.
The Distance May Vary
The Joint Commission, which provides standards for hospitals in the United States does not have a recommended distance that a smoking area should be from a building. Instead, they recommend that several factors be taken into account including the location of windows and vents, the type of doors and other factors that can effect whether or not smoke enters the building.
When determining the distance at which to place a smoking area from a building's entrance you should take into account the specifics of the building, but the Joint Commission also recommends that you should follow any local or regional legislation that prescribes a distance that smoking areas must be from an entrance.