I Have A 4 Year Old Male Dachsun.... He Has

I have a 4 year old male dachsun.... he has not eaten his food for three days now. He is active, drinking water and eating a dog biscuit in the morning, but will not touch his dinner. He is being fed Purina dog chow which was recommended by my vet. I was mixing 1 tbsp of vanilla yogurt in his food because of his breath and my vet said to try this as it will help if his stomach is causing his horrible breath. He was eating that really well until three days ago... I have tried no yogurt in his food and just a little water in his food. No go. How long can he go without eating?