Generally speaking, you can candle at any time after 3 days, but most experienced poultry people have their own preferences as to when they like to candle.
Personally, I like to candle twice -- once on day #7 and once on day # 18. By day #7, you can pretty easily tell which are growing and which are infertile, so you can get rid of the infertiles. On day #18, you can usually tell the difference between the live eggs and those that have died along the way, and get rid of the dead ones.