The rules say to keep and seed your uploaded files until at least 3 or 4 other members are seeding your torrent.
But this limit is not good for a better ratio. Experience shows there are several phases in torrent life: Directly after upload where a lot of members try to get the new download and keep seeding it for some time. After this period there comes a calmer time for seeding - slow but rather secure for uploading. If you are lucky your torrent will get promoted to the top of the torrent search - with even more luck (or your seedbonus points) the torrent will get freeleech - in that case you are really lucky. Another tip is to watch for re-seed requests for your download - those re-seed requests are secure way to get upload traffic.
As to harddisc place: you can also burn the data files (unchanged!) to CD or DVD to free up your hard drive.
Hope this hekps.