"They" used to say... like a dollar bill. Ideally very thin, if you get everything working correctly. When you achieve an Athletic setup (centered and balanced), you should be able to swing every time and just barely brush the ground. When you get the ball in the correct position for "your" swing and the club you're using, you should be meeting the ball at or just before the bottom of the swing arc. Play around with ball positions. You might have the ball too far back and when you make contact, your club head is still descending and continues its path downward, gouging out a thick divot. Sometimes, golfers drop their chest* on the downswing causing the club head to dig deeper than they originally had setup.
* Many times this is due to the fact that they raised up on the backswing, so (for every action - opposite and = or more reaction), they drop their chest on the downswing causing radius issues with their swing.
Move Less ... Get Good! DUPLESSISGOLF