You can always candle anytime you want to or feel the need, but the standard times are day 5 (to see if there is developement...the swirly speck with veins surrounding it like the eye of a hurricane), then day 10 to see if there is progression (usually half dark shell, big air space, large veins going along side), and then sometimes at day 18 to make sure there is still development, and then at day 22 or 23 to see if there is still hope.
I typically only check around day 5, sometimes day 10, almost always at day 23 if nothing has happened....unless dark colored eggs like I stated earlier...those I can only see if they got some sort of start and then at day 23 if they haven't hatched to see some sort of sign of life.
Lady of McCamley