It extends 3 minutes from the last bid. So if somebody else bids within the last 3 minutes then it extends again. It only runs out when nobody bids within the last 3 minutes. This isn't necessarily unlike live auctions though. If the auctioneer says "going once, going twice..." and then somebody bids, the person that bid at the last second does not win the item. The auctioneer then tries to get another bid before going into the final countdown to the gavel. Seems like BB is closer to real life than most online auctions.
Just because this site does not function like Ebay does not make it a flaw, it just makes it different. Also another prominent auction site, Quibids.com, adds 20 seconds every time someone makes a bid...seems similar to BB to me. So it sounds like I could characterize this complaint as a matter of preference and not as a flaw.