You should park your car less than six inches away from the curb.
Within 18 inches.
Here in California it's 18 inches.
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 if there is no curb.
I don't know how much it varies from state to state but it was "within 6 inches" when I took my test many years ago.
the maxium is 6 inches but it has been debated by many if it is more
The distance is likely to be different depending on what State you live in. Where I work, in the State of Washington, the front wheels must be within 12 inches of the curb, and if on a hill, the front wheels must be turned in.
Under laws bought in in 2009, you may be fined you if you park more than 19 inches from the kerb but this is only enforced by some local authorities.
18 inches
18 inches
2-18 inches usually.
12 or less inches