When posting items to the auction house you have the option to list the items for 12, 24, or 48 hours. When I post items to the auction house I post all my items at once and post them for 24 hours at a time usually at around 10pm. It can take me up to 15-30 minutes to post 200-300+ items. This allows me to setup a schedule and a known time for when I have to log into the game and re-post any items that did not sell.

I try to stay away from posting items for 48 hours because the market can often fluctuate and you may find that posting an item on Monday for 48 hours at 100g that item may only be worth 50g by Wednesday and many other people will more then likely have under cut you. I post all my items for 24 hour periods because I can log in once a day and re-post any item that did not sell on a fixed schedule.

I try and stay away from posting items for 12 hour periods because it would require me to spend another 15-30 minute session re-posting items. You have to balance out how much time you are willing to spend managing the items you have on the auction house verses how much time you plan on actually playing the game or raiding etc.

Posting an item for 12 hours is good for when you want to undercut your competition and almost always have your item listed as the cheapest item available. But it requires that you re-post the items every 12 hours or twice a day. 

The idea here is to ensure that you always have items on the auction house for sale at all times. If you schedule your posts every 12 hours at say 8am Monday morning but you forget to log in and re-post the items later at 8pm Monday night, you will probably not get around to re-posting those items again until 8am Tuesday morning. That means that your items were not listed on the auction house for a full 12 hours resulting in a potential loss of sales during that time.