Actually, now that I've said that...
Usually I am able to able to see changes in the coloring of most of my Guinea eggs (if they are fertile) as early as 4 days into the incubation, but I've looked at a LOT of eggs so the slight change is fairly easy for me to see. Some eggs are too dark shelled or even just super thick shelled to see thru well enough and I wait til the 7 day mark on those. Anywhere from 7-10 days is usually when development or no development, and even blood rings are all pretty obvious tho.
Usually I am able to able to see changes in the coloring of most of my Guinea eggs (if they are fertile) as early as 4 days into the incubation, but I've looked at a LOT of eggs so the slight change is fairly easy for me to see. Some eggs are too dark shelled or even just super thick shelled to see thru well enough and I wait til the 7 day mark on those. Anywhere from 7-10 days is usually when development or no development, and even blood rings are all pretty obvious tho.