It could be 24 hours. It gets really exciting doesn't it, but then the wait is killing!
They will emerge altogether, it is amazing how they know. The first hatchling will wait until the last one is ready to say hi!
Be very careful at the moment - it is too cold for chicks at the moment. Best to keep them in a barn so they are out of the elements.
Our chooks went broody very late this year, infact one has just gone broody now but I cannot leave her with any eggs, cruel as it sounds - they just wouldn't make it now. I have two clutches which are two weeks apart. The first hatchlings are coping fine, because they have size on their side and they had all the lovely warm weather. The second clutch have been hit and miss. One died one night, three are doing find and I have one naked neck chick which just cannot cope outside. I bring it in to warm it up and it gets all lively - return to mum - within an hour it is all feeble again. It is about a third smaller than the other chicks. After three days of trying, I have finally decided to keep it in and hand rear it in the bathroom. It is right next to the fire at the moment, and will have to stay there until it puts on a bit of weight and size.
Good luck with yours anyway, and be patient!
Kate