I mostly ride in the center of my lane, so, for me, it's half a lane; the driver should be in the next lane. I could get by with a few feet, but riding far to the right is an invitation to get clipped, and it's a choice between half a lane, or half a foot, which is an easy choice.

Also, I like leeway. A strong gust, debris in the road, or a suicidal squirrel can send a car or a bike lurching more than a foot leeward, and you can't really see the wind while you're cycling.