So, if a Roo does his thing today..........?

I think it takes two or three days for the eggs to be fertile, from what I have read on here.

 

They can be fertile, but that isn't usually the case. More often than not the hen doesn't trust the roo enough to fully submit or he just doesn't have the skills yet. I give at least a week before I collect.

 

Depends on the roo also - I have a GLP roo, a teenager, and he is new to my flock of year olds... He "does his thing" but isn't quite making the proper contact yet. Cochins are just too much hen for him!

 

In answer to your question, if the rooster is successful, maybe two but probably three days. You can crack the eggs and check to see if they are fertile. This might help with that.

https://www.backyardchickens.com/forum/viewtopic.php?id=16008

In answer to a question you did not ask, the eggs will probably be fertile for two weeks. Could be a little less and could be more. I've seen posts where eggs were still fertile after 3-1/2 weeks, but this is an exception and certainly not to be counted on.