If I give you earnest money to hold something how long are you required by law to hold it

Although it is possible that GA law is different, there would be no legal requirement as to how long you must refrain from selling property after accepting a deposit.  It depends on what the contracting parties agree on.  Did you agree with the owner that he would hold the camper for a specified period of time?  If this term was not discussed, the law will generlaly imply a reasonable amount of time, but "reasonable" depends on the circumstances.