If you were the initial seeder (uploaded a torrent), then you should keep it up as long as you can. But once other people have downloaded it to 100%, and are willing to seed you can delete it.
If you DOWNLOADED the torrent, then after you are done seeding for however long you want, then you can delete it too.
Dont just download then delete, or you are a leecher. The point of torrents is to help out and seed for as long as you can.