It also depends what you mean "last". After a few weeks in the fridge, to me they are noticeably "not fresh-laid" but still quite perfectly good to eat. After a couple months in the fridge, they are a little sulfury and you might reserve them for baking rather than eating sunnyside up, but will mostly not kill ya. After 4-5 months in the fridge, they may not kill ya but you probably won't be in a hurry to want to eat them either
All times reduced for eggs kept unrefrigerated on the countertop, of course (and there, keeping times depend SO much on what temperature your kitchen is!)
The USDA says that eggs are safe if used within 3-5 weeks. Here is the link that has everything you every wanted to know about egg safety: http://www.fsis.usda.gov/Fact_Sheets/Focus_on_Shell_Eggs/index.asp Eggs in my house never last a week, but we sell most of them and I do a lot of baking etc. with the ones I keep. This site says even eggs that float (large egg cell) may still be good. Just crack into a separate bowl and check to make sure there is no bad smell or green tint.
When I sell my eggs I include the lay date (I use a sticker so I can reuse cartons) and the "use within 3-5 weeks". I'm still trying to figure out why the 2 week difference in freshness though. It probably depends on how someone stores and handles them. I do not wash my eggs (cold water can damage egg worse than lukewarm water but I still don't chance it). I use a stiff brush or fine grain sandpaper (220 or higher). If I get and egg that is really dirty (it happens sometimes even though I keep the nests really clean and replace bedding material often) I just toss it. I have enough layers that it's no big deal for me.
Here is another link from culinary experts on eggs: http://forums.egullet.org/index.php?/topic/36558-all-about-eggs-introductory-materials/
Hope all this helps.