How long does it take for a torrent to download

The torrent itself is only a small text file that takes only seconds to download.

I assume your question is about the content associated with a torrent file and that varies greatly.

There are several factors involved:
The size of the content. Torrents vary greatly in size. Obviously a 30MB download has a different download time than a 3GB download.
The speed of your internet connection. A 28kbps dial up connection is not going to download as fast as a broadband connection of 20Mbps.
The upload capacity of the individuals active on the torrent. Your download speed on a torrent depends on the upload speed of the other active users on that torrent. Bittorrent distributes content using the upload bandwidth of active users.
The set up of your bittorrent client. A properly set up bittorrent client (one that has a clear network connection and upload correctly capped) will download much faster than one that is not properly set up.

So, the answer to your question is "as long as it takes".