Every year. Dogs are required by law to have Rabies shots - they can be 1, 2 or 3 year shots, so you'll have to ask your vet which one you got to see when he needs a new one.

Dogs also need Parvo and Distemper, and whatever other vaccines are needed for your area. Those are all 1 year shots I think, so your dog should go every year for those.

And you especially want to make sure he's updated on shots before you bring your new dog home (hoping that you get him! ^_^)