We just candled our batch of duck eggs at Day 5, and with a good LED flashlight, it was really clear what was growing, and what wasn't. There was a very definite dark spot and veining on the viable ones, and one that was perfectly clear. We felt very comfortable tossing that one-- it was just so obvious that nothing was growing, and the egg likely just wasn't fertile.
A lot of people here recommend waiting until Day 7, though, which is also a real safe bet if you're not confident in your flashlight, or in your candling abilities!
As far as when to toss, it depends on what's going on with the egg. Most recommend tossing immediately any eggs that are weeping or that smell. Otherwise, the usual recommendation is to keep it unless you're absolutely sure it's no longer viable-- better safe than sorry!
The complete series of pictures that Silkiechicken posted in the beginning of that thread are completely invaluable for all first time hatchers. And in my humble opinion, should be required reading/viewing for all first timers! I know it sure helped me with all of my candling questions!
Be sure to watch the videos too! Just amazing!! I positively squealed with delight the first time I saw movement in my own eggs!! It was exactly as depicted in the video! All first timers owe a great debt of gratitude to Silkiechicken for this amazing achievement in documenting in pictures, the progression of development in eggs!!
To answer your question though...wait until day 10 for your first candling. The good eggs will be so blatantly obvious over those that are quitters. If you have the least bit of doubt, keep it in the incubator. I had 4 such eggs that I wasn't sure about my first time. I left them in until my second candling on day 17. At that point I was certain that two of the 4 had quit fairly early on. I was still doubtful of the other two but left them in anyhow. I'm SO glad that I did, because 3 days later, they BOTH hatched!!
I always candle at day 5. You should be able to see what looks like a red spider by now....good LED flashlight is worth its weight in gold ! I also candle at day 12 and the day before lockdown....if there arent any visible veins or movement when I candle on day 12 I toss them. Hope this helps! Good luck on your hatch!