Collect the number you want in 5 to 7 days then start incubation. Most people will say not to hold them much longer than 7 days. Store them in a cool place in your home( not the fridge) with the small end down. Good Luck!
if you have a rooster. then collect them and put save them in a warm dry area for about 5 days then you can put them in the bator( I do this cause I only have bout 5 hens that are laying).. If you have a lot of hens and roosters then you could put them in the bator the day they are laid..