I seed until I run out of space on the ol' 1 TB external drive, then purge what's been watched once a year or so. The super-popular hundreds-of-seeders torrents for recent series might get cut out sooner. Stuff on my laptop or desktop's hard drive is seeded until the computer dies, pretty much. The rare, obscure, or annoying to find things get the privileged spot on the computer itself, because they're the torrents that took me months or even over a year to complete off of forgotten trackers. So, basically... I prefer to hang on to the poor, unloved torrents to pass on the love to future scroungers.
The Midori movie was a bitch to find when I snagged it in 2007, and I was the lone seeder for Grimm's Fairy Tales Classics for ages because it took me a year and a half to finally get all the missing pieces and didn't want anyone else to go through that. Now they're both here at BakaBT, so it's a non-issue and I like helping to keep it that way. -Because nothing is more frustrating than finding exactly what you want, and it has no seeds.