How long do 7GB torrents take to download?

That depends on the speed of your internet connection and the health of the torrent.

If you see that you are getting a constant speed on the torrent, you can calculate in Google the amount of time.

So if you are getting an average download speed of 100kBps (bittorrent clients show speeds in Bytes), you would enter this in Google:
7GB/100kBps
and get 20.3889778 hours