How long was a slave auction?

It depends on what sort of Slave Auction it was, the "grab and go" and the highest bidder auction, usually after the ships carrying slaves would dock the slaves were cleaned and inspected. The highest bidder auction was where the slave was put on a platform, inspected of and bid for the highest price (So whenever the last slave was sold is how long it was) the other "grab and go" was quick, more of a purchasing of a raffle ticket and then the buyers were able to run into a pen and fetch the slave of their choice.