Here in California it's 18 inches.
the maxium is 6 inches but it has been debated by many if it is more
From the Calfornia DMV web page on parking: When you park alongside a curb on a level street, the front and back wheels must be parallel and within 18 inches of the curb. Park parallel to the street...
1,557.72 kilometres (967.93 miles)
In almost all areas the distance is at least 15 feet.