~50x
My policy is double my download, minimum. But I almost always hit much higher than that, my average is probably between 5-10x
All my torrents are public sites, and I only torrent pretty common stuff, so I don't feel too bad about killing a torrent after a week or two. I figure 5-10x average on easy-to-find, mid quality media is plenty in the karmic sense lol.
As far as I am concerned, always give better than you get, even if that's 1.01 but try to aim higher.
Of course, if you're seeding a rare or otherwise hard to find piece of media, then you should keep it alive for longer. I am in the process of upgrading my torrent machine, and once that happens, I will be able to hold far more active torrents, and my average ratios will be significantly improved.
Happy sailing!