I'm not big on downloading old content at slow speeds without much chance of seeding it back. I like being part of an active swarm. I think we should just let a lot of torrents die sooner than those dedicated seeders would allow (if you have more than 100 torrents older than two weeks in your client right now, I mean you).
I'd say 1 month is a good period.
But, I'm curious, what do you all think?