The measurement you need to be concerned with is the distance from the rear finished wall and the center of the drain. It should be 12 inches. 12 inches is standard. if you have more or less you can get a 10" or 14" but they are not the norm.
To add to Jim's comments, which are correct, if it's a little more than 12" from the center of the drain you can probably still use a standard 12" rough toilet. You just don't want the back of the tank to be much more than about an inch away from the wall, so 13" is probably about the max. If it's much less than 11 1/2" you will need to buy a 10' rough in model.