We have found a local health food store that is willing to buy and sell our eggs. We need to stamp the cartons with exp. dates. How long is the shelf life of a farm fresh egg?

Please check the law. Here, one has to have a license (to candle and grade eggs) in order to sell to retail outlets (stores, restaurants)

Who is requiring the expiration date? If the law requires it, then the time frame should be laid out by the law and not subjective. It would also be dependent on washing and storage procedures.

Find out the specific laws where you live. I found the law for my state online after a little digging.