Good question. I don't have a simple answer though.
It depends. What causes an egg to rot is that bacteria gets inside and multiplies. The hen puts a coating on the egg, called bloom, which inhibits bacteria from getting inside. It’s not perfect and if the egg is dirty (especially damp-dirty with poop or mud) bacteria can still get inside, but it works pretty darn well. Hens can lay eggs in nests for two or more weeks, sit on then for three more weeks, and hatch them without bacteria getting inside. Happens all the time. Eggs can last months on your kitchen counter if bacteria does not get inside.
How fast will that bacteria multiply once it gets inside? Depends on the temperature. If they are refrigerated it could take a long time. If they are at incubation temperature, well that is a perfect temperature for them to grow. If the temperature is somewhere in between, then the rate will be somewhere in between. It’s not a clear answer.
Are the ones you found safe to eat? Probably, but it’s not assured. How many days has one been laying out there?
There are a couple of things you can do to check them out. First, just sniff them. Do you smell a rotten egg smell? If you do, very carefully get them out of your house NOW! (yes, I meant to shout)
You can try the float test. This does not really tell you if they are good or bad, but it gives you some clues. Eggs lose moisture the longer they sit. If you put a fresh egg in a bowl of water and it lays flat on the bottom, it is pretty fresh. If it stands on end but stays on the bottom, it is a little older. It has lost some air and the air cell has grown some. If it floats, it is older and more suspicious. That does not mean it is bad, just that it is older and should be treated with more suspicion. In any case, crack them in a separate bowl before you use it to check it out. If it has gone bad, you’ll smell it.
If bacteria is inside the egg, it will give off a gas. That can cause an egg to float too, even if it is not all that old. Certainly treat any egg that floats with suspicion.