At day 4 you can start to see veining, though faint. Day 10 you will know for sure if they are developing as it will be nearly solid black inside except for the air pocket.
Candle on Day 7 for the first time is what I understand. I let mine sit at 70F for 20 hours before I set them. I'm not sure what could happen if you don't. Sure someone will pipe in. You can always do a search. It's amazing how much info is on this site from past posts!!!!
I usually only let mine sit a few hours. I remember once I put them right in and they did hatch.
If you are new to candling, I would not do it until after Day 7, JMHO. The vein network is very delicate before that and just turning the egg to try to see can damage the veins. I use a slide projector or a super bright LED flashlight. A regular flashlight isn't the best thing to use, has to be one of the really bright ones.