If I have to pick just one I pick two weeks. Two avoids clutter on vendors and fights lag.
But the better option is options. Put in all the above if possible and let the saler pick how long they wish the iteam/s to be up for sale.
Though I also belive there should be a time limit and never Unlimited or till end of sub. It will in the end just make to much lag. Fight the lag limit it to no longer then 90 days.