Annually. We live in a heart-worm area, and have the annual heart worm injection. there is also the risk of parvo (which is known to be carried in the local dog-pond), which DH's corgi had when she was little because he was very dim on vaccinations (didn't know they were needed).

I'm not worried about Kennel Cough as regardless of vaccination, they manage to pick it up anyway. And like the flu vaccine, it comes in multiple strains, and you're relying on your dog coming in contact with the strains you vaccinated for. Never our luck! But our dogs don't get really ill from it anyway.