you go to rehab when the judge tells you it's that or jail.

edit:

Weed is not a physically addictive substance. That's like going to rehab for cigarettes. While you CAN go, because it's a drug and illegal, you probably won't get much out of it. They brain-wash you with programs not far from the 12-steps, and completely fuck with your head. Unless you honestly believe that pot is controlling your life, counseling or even just a smoke break would be more effective and less costly.