Field grade article 15 in AIT and I'm a National Guard Soldier?

It all depends on what they want to do with you. Extending you ten days means they have to pay you for ten more days even if you are doing extra duty during those days. Don't be surprised if they just make you do extra duty until the day before you leave. Be glad they're letting you finish AIT instead of kicking you out.. Article 15s don't come up on a civilian record but when you are going through your career it will be on your military record. Consider this a lesson learned, people in the Army don't tolerate thieves and I've seen barracks thieves have a really bad day when they were caught and I don't mean with the chain of command.