Depends on the temperature - if it's really hot, not more than a day or two. If it's cold, they can last much longer, like a week. Also, if you have a rooster, keep in mind that hot temperatures can start the eggs developing.
100 degree days are pretty common here in CA. Just got home last night from a weekend away - I collected eggs and since it's been in the 90's all weekend, I cracked them so I could throw them away and check fertility. I had about a handful that had started development. Not an embryo yet, but still developing.