It is really hard to say for sure. You are getting close to 1 week essentially without food and ducks can potentially deteriorate quickly. While they hate tube feeding and it is stressful for both of you the first time, it is better than loosing your duck. I can say that from personal experience. I would get a family member or friend to help you with it. It helps a lot the first time. Tube feeding only last a minute or two. You slide the tube down into their throat, and pump fluids in. 1-2 hours later you do it again, but you pump the food in. It is stress on the animal for a few minutes, which is well worth their life. It will take a bit more time the first time, making sure he is breathing, and such.