timeout ssh

SSH Client

This method is the way to go if you have no administrator access to the server you are connecting to. This method will apply to all the servers you are connecting to, instead of only to a specific server.

What you're basically to do is to configure your SSH client client to periodically send keep alive message to the SSH server. If you're running Ubuntu / Debian, edit /etc/ssh/ssh_config and set ServerAliveInterval option to the following;

ServerAliveInterval 100

This option is to tell your SSH client to automatically send the keep alive message every 100 seconds to the SSH server, even if you're away from your client machine. The server will assume you're not idling and will not disconnect your session.
SSH server

If you have administrator access to the server, you can configure the ClientAliveInterval, TCPKeepAlive and ClientAliveCountMax options in the SSHd configuration file. If you're running Ubuntu / Debian, 
the file's path is /etc/ssh/sshd_config

ClientAliveInterval 30
TCPKeepAlive yes 
ClientAliveCountMax 99999

You will need to restart the SSH server for the changes to take effect.

sudo /etc/init.d/sshd restart