Chocolate melts below 88F. Ganache melts below that by a few degrees depending on how you made it.
But how long it takes to melt depends on the size of cake and the room temperature. A 14" cake that has been chilled overnight can last about 6 hours at 86F before it is too soft to be stable. Using supports/dowels properly will extend the stable time of a stacked cake.
You will see some posts here and there that bakers refuse to permit their cakes to be placed where the temperature is over 80F. This is for food safety reasons as well as to protect the stability of the cakes.
Being cautious about temperature exposure is always an intelligent approach. Parties do not always go as planned, and one must always allow for a few more hours of room temperature exposure--starting with the delivery when the site is not open as promised.
One cannot totally prevent a disaster if a guest should stumble near a cake that has become soft. One can insist that the cake remain chilled onsite before the order is placed, and then the responsibility lies with the customer.