It depends on what kind of shell you are dealing with as well as how powerful your light is. I'd wait till day 10 to make any culls.
On darker eggs esp, it can be day 10 before you can see anything.
If you are new to candling it doesn't hurt to error on the side of caution.
As long as none of the eggs smell bad or are weeping fluid you could do a another candling at day 18, then it's really easy to tell if there is a chick in there.
They will basically fill up the egg except for the air cell.