I don't use BitTorrent anymore, but when I did my rule of thumb was to have at least a ratio of 1.0, or continue seeding it for a month, whichever happened firs

I don't use BitTorrent anymore, but when I did my rule of thumb was to have at least a ratio of 1.0, or continue seeding it for a month, whichever happened first. After which I would feel fine deleting it.