Are you talking about chicken eggs, or duck eggs? Well here's what I know about chicken eggsnot duck eggs)
After the egg is laid, wait 4-7 days. By that time, veins should be visible. If by day 10 the eggs are not showing any signs of development(veins, a shadow in the egg, etc...) then you should remove it. Usually what people do to an egg that isn't developing(AFTER WAITING UP TO 7 DAYS) is they make a hole on the top of the egg where the air sack is and see what is in the egg. Some people(like me) want to find out why it didn't develop or if it was even fertile.
This is about the same way I do it, except I don't bother candling at day 18, I can never see anything anyway at that stage. I used to candle earlier, with my first hatch, but I could never see anything, so I figured why bother? But I do always give the eggs the benefit if the doubt. If I don't see anything the first time, I put them back, and wait till the 2nd before tossing. If you can't see anything by then, then there is definitely nothing there.
It's when they start oozing... looks like clear sap starting to push through the shell, but it hardens quickly in the heat. So, if the eggs are all clean when you put them in there, then one day you look and it looks like someone dripped something on the eggs, they're weeping, and are no good.