How long to wiat before trying to hatch

My hens (black australopes), have been laying for about a month. I am thinking about trying to hatch a few just for experience. The eggs should be very fertile with 3 roo to 7 hens. How long should the hens be laying before trying to incubate some of their eggs?

Also, I know it would be somewhat unusall, but I have one or two hens laying two eggs a day. We mark the eggs each day and have been keeping close count, but somedays we get eight eggs and some days 9. We only have 7 hens and it isnt likely someone is slipping extra eggs in the nest for a prank. Has anyone ever heard of a hen laying more than one egg a day?