API AuctionFrameAuctions.duration

AuctionFrameAuctions.duration = time in minutes
 //set the short duration button to be checked and set the time
 AuctionsShortAuctionButton:SetChecked(1);
 AuctionFrameAuctions.duration = 120;
 
 //set the medium duration button to be checked and set the time
 AuctionsMediumAuctionButton:SetChecked(1);
 AuctionFrameAuctions.duration = 480;
 
 //set the long duration button to be checked and set the time
 AuctionsLongAuctionButton:SetChecked(1);
 AuctionFrameAuctions.duration = 1440;