For a 12 year old who was caught in a group vandalizing property, I would:
Make my child go through the entire police process - no getting a lawyer to bail him out (although I would not allow any time alone in a jail setting) or trying to plea out the offense.
Make my child earn the money to repay his or her portion of the damage.
Make my child write letters to the property owners and the poilce officials apologizing for the poor decision and for the expense and time it took away from more important matters.
Sit down with my child and go over every activity they enjoyed - and set the limits I wanted to. I might allow books and a family movie once a week, but no going to movies with friends, etc. I would want to be in control of all of those activities but I would never do a blanket grounding because I think there would probably come a time when I would likely make an exception to something and I would want to give myself the "out." If I caved once for something I felt strongly about, the next time my child could use that to say, "But you let me...". Better not to set up the expecation in the first place.
Even for this kind of behavior, a month would be as long as I would be willing to deal with punishment. After that, what more could I really hope to accomplish? Either the kid has learned a lesson and I'll have a follow-the-rules-forever kid on my hands, or he's blown it all off, in which case we'd be moving on to the next phase and seeing a professional for counseling.